Budding yeast as an ideal model for elucidating the role of N6‐methyladenosine in regulating gene expression

N6‐methyladenosine (m6A) is a highly abundant and evolutionarily conserved messenger RNA (mRNA) modification. This modification is installed on RRACH motifs on mRNAs by a hetero‐multimeric holoenzyme known as m6A methyltransferase complex (MTC). The m6A mark is then recognised by a group of conserved proteins known as the YTH domain family proteins which guide the mRNA for subsequent downstream processes that determine its fate. In yeast, m6A is installed on thousands of mRNAs during early meiosis by a conserved MTC and the m6A‐modified mRNAs are read by the YTH domain‐containing protein Mrb1/Pho92. In this review, we aim to delve into the recent advances in our understanding of the regulation and roles of m6A in yeast meiosis. We will discuss the potential functions of m6A in mRNA translation and decay, unravelling their significance in regulating gene expression. We propose that yeast serves as an exceptional model organism for the study of fundamental molecular mechanisms related to the function and regulation of m6A‐modified mRNAs. The insights gained from yeast research not only expand our knowledge of mRNA modifications and their molecular roles but also offer valuable insights into the broader landscape of eukaryotic posttranscriptional regulation of gene expression.


| INTRODUCTION
Chemical modifications of RNA play critical roles in regulating gene expression.More than a hundred RNA modifications have been identified.Ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs) have been shown to be the most modified species of RNA (Barbieri & Kouzarides, 2020;Jones et al., 2020;Karijolich et al., 2010).Numerous messenger RNA (mRNA) modifications have been identified, and their importance has been demonstrated across various stages of the mRNA life cycle (Gilbert & Nachtergaele, 2023).mRNA modifications vary in abundance, act on many transcript types, and have various roles in controlling gene expression (Barbieri & Kouzarides, 2020;Gilbert & Nachtergaele, 2023;Jones et al., 2020;Linder & Jaffrey, 2019).Many mRNA modifications are highly conserved from yeast to humans (Höfer & Jäschke, 2018;Karijolich et al., 2010;Tardu et al., 2019).Among all mRNA modifications, N 6 -methyladenosine (m6A) is the most abundant internal mRNA modification (Liu et al., 2022).The regulation of the m6A-modified mRNA and the effect m6A exerts on the fate of mRNAs have been subjected to intensive research over the last decade.Studies in higher eukaryotes have shown that the m6A modification regulates gene expression posttranscriptionally.Specifically, m6A has been reported to regulate splicing, export, localisation, translation and decay of mRNA (Gilbert & Nachtergaele, 2023).Misregulation of the m6A modification can have a substantial impact on the overall physiology of an organism, including defects in cell differentiation, altered stress response, effects on immunity, and developmental processes (Lasman et al., 2020;Wang, Cai, et al., 2022;Wang, Zhuang, et al., 2022;Yang et al., 2021).Moreover, the m6A machinery has been shown to be a potential therapeutic target for cancer treatment (Delaunay et al., 2023).
In budding yeast, Saccharomyces cerevisiae, the m6A mark is specifically deposited on mRNA during early phases of meiosis where it is important for meiotic progression (Agarwala et al., 2012;Ensinck et al., 2023;Schwartz et al., 2013) (Figure 1).The yeast methyltransferase complex (MTC) that deposits the m6A modification on mRNAs and the YTH domain-containing protein, Mrb1/Pho92, that associates with m6A are highly conserved.In this review, we will explore the unique advantages of yeast as a model system for understanding the fundamental mechanisms of gene regulation mediated by m6A.We will cover the recent findings on the functions governed by the yeast MTC and Mrb1/Pho92 in yeast meiosis.

| BUDDING YEAST AND FISSION YEAST HAVE DIFFERENT m6A METHYLTRANSFERASES
Budding yeast was one of the first models used to show that m6A has an important function (Bodi et al., 2010;Clancy, 2002).The IME4 gene (Inducer of Meiosis 4), which encodes the sole m6A methyltransferase in budding yeast and is orthologous to METTL3 in humans, is not essential for viability and acts specifically in the early phases of meiosis (Agarwala et al., 2012;Ensinck et al., 2023;Schwartz et al., 2013).The tight regulation of m6A deposition during early meiosis in budding yeast contrasts that in higher eukaryotes, where m6A is constitutively and by default deposited (Dominissini et al., 2012;Uzonyi et al., 2023).Also benefiting from the classical advantages as a genetic system, budding yeast serves as an excellent reductionist system to study the function of the m6A mark in mRNAs (Botstein & Fink, 2011).
In Schizosaccharomyces pombe, fission yeast, the U6 small nuclear RNA (snRNA) is m6A modified by Mtl16, orthologous to METTL16 in humans (Ishigami et al., 2021).The U6 snRNA m6A mark regulates pre-mRNA splicing in a genome-wide manner.The fission yeast genome also harbours a gene annotation named ime4, which likely derived its name due to the conserved MT-A70 domain.Phylogenetic analysis of methyltransferase domains (MT-A70) revealed that the MT-A70 domain of fission yeast's Ime4 evolutionarily belongs to the METTL4 subfamily, and not to METTL3/Ime4 budding yeast subfamily (Yue et al., 2015).METTL4 catalyses the 2′-Omethyladenosine (Am) to generate internal N 6 , 2'-Odimethyladenosine (m6Am) modification on U2 snRNA to regulate splicing and has been shown to be implicated in methylation of mitochondrial DNA to generate N 6 -methyldeoxyadenosine (Chen et al., 2020;Goh et al., 2020;Gu et al., 2020;Hao et al., 2020).

| BUDDING YEAST AS A MODEL FOR m6A METHOD DEVELOPMENT
When the field of m6A biology took off, budding yeast remained at the forefront but was mostly used for technological developments likely due to its transcriptome being less complex than mammals.
More importantly, it is relatively straightforward to completely abolish m6A in budding yeast.The ime4Δ strain has been used as a critical negative control for generating transcriptome-wide maps of m6A-modified mRNAs using m6A-seq, a technique by which fragmented mRNA are immunoprecipitated with m6A antibodies F I G U R E 1 N 6 -methyladenosine (m6A) is present in early stages of yeast sporulation.Scheme of the sporulation programme.Indicated at the stages of sporulation where m6A deposition starts, peaks then declines.Also indicated are representative protein level of methyltransferase complex component Slz1 and YTH domain-containing protein Pho92 throughout sporulation.

Take aways
• Yeast is an exceptional reductionist system for exploring the impact of m6A on the regulation of gene expression.
• m6A is selectively deposited onto messenger RNA (mRNAs) by the conserved methyltransferase complex (MTC) during the early stages of meiosis.
• A noncatalytic function of the MTC is also critical for meiosis and sporulation.
• Pho92, the sole and conserved m6A reader protein in yeast, plays a key role in promoting the decay of m6Amodified mRNAs.
followed by deep sequencing (Dominissini et al., 2012;Schwartz et al., 2013).One of the first accurate genome-wide maps of the m6A transcriptome showed dynamic m6A regulation during yeast meiosis and included the ime4Δ mutant, which proved to be critical for precise mapping of m6A-marked mRNAs (Schwartz et al., 2013) (Figure 1).New iterations of m6A-seq were also developed using yeast including an antibody-independent method of m6A detection called MAZTER-seq, m6A-seq2 for multiplexing of samples, and more recently m6A detection by Nanopore direct sequencing (Dierks et al., 2021;Garcia-Campos et al., 2019;Leger et al., 2021).Transcriptomewide mapping showed that the m6A mark is present on over 1000 transcripts, specifically in early meiosis (Figure 1).Similar to higher eukaryotes, m6A is predominantly present at or near the 3′-end of transcripts.However, there are examples of transcripts where m6A is present at the 5′-end and in coding regions (Schwartz et al., 2013;Varier et al., 2022).Moreover, the m6A motif sequence is comparable to that of higher eukaryotes.With that in mind, it is perhaps not surprising that budding yeast as a simple eukaryotic model is highly suitable for studying the fundamental molecular mechanisms of gene expression regulation by the m6A mark.

| THE YEAST MTC IS CONSERVED WITH DISTINCT FEATURES
The m6A modification is installed on mRNAs by the mRNA methyltransferase complexes (MTCs), also known as the m6A writer complexes.MTCs typically prefer the RRACH/DRACH motif sequences in mRNAs for depositing m6A and require the cofactor S-adenosyl-L-methionine (SAM) as a methyl group donor (Tuck, 1992;Watabe et al., 2021;Zhou & Pan, 2016).In yeast, Ime4 is the catalytic subunit and is essential for m6A deposition.Cells lacking IME4 have a severe delay in meiotic progression and display severely reduced spore formation in the sporulation-proficient strain background SK1 (Table 1).However, in other common lab strain backgrounds, for example, S288C and Σ1278b, IME4 is required for meiosis and spore formation (Clancy, 2002;Hongay et al., 2006;Shah et al., 1992).Cells that harbour the catalytically inactive Ime4 (IME4 CD ) display a much milder meiotic phenotype indicating that Ime4 possesses an important noncatalytic function in meiosis (Agarwala et al., 2012;Bushkin et al., 2019;Ensinck et al., 2023;Park, Belnap, et al., 2023).Two additional proteins, Mum2 and Slz1, were identified to interact with Ime4.The Mum2, Ime4, and Slz1 complex, also known as the MIS complex, was one of the first MTCs described in eukaryotes (Agarwala et al., 2012) (Table 1).
The MAC-MACOM complex exhibits greatly enhanced methyltransferase activity compared to MAC alone (Su et al., 2022) (Figure 2a).
Recent works showed that the yeast MTC contains three additional subunits (Kar4, Ygl036w/Vir1 and Dyn2).Furthermore, the new analysis showed that the yeast MTC is much more conserved than previously anticipated (Ensinck et al., 2023;Park, Belnap, et al., 2023;Park, Sporer, et al., 2023).Briefly, Kar4 was identified as the METTL14 orthologue.Ygl036w was renamed to Vir1, because its sequence is orthologous to VIRMA/Virilizer (Ensinck et al., 2023;Park, Belnap, et al., 2023;Park, Sporer, et al., 2023) MTC via Slz1, has no clear role in m6A deposition.Slz1 has sequence similarities to ZC3H13, while WTAP is the mammalian orthologue of Mum2 (Agarwala et al., 2012;Ensinck et al., 2023).An earlier study showed that disruption of SLZ1 decreased m6A by 80% (Agarwala et al., 2012).However, a recent study reported that no m6A levels were detected across different phases of early meiosis in slz1Δ cells (Ensinck et al., 2023).It is worth noting that slz1Δ cells also display a mild meiotic phenotype comparable to that of IME4 CD .In summary, five of the yeast MTC subunits are conserved in higher eukaryotes (Figure 2a).No orthologues of HAKAI and RBM15/15B have been identified in yeast yet.
Even though subunits of the yeast MTC are largely conserved, the yeast MTC displays distinct features compared to higher eukaryotic counterparts.transcription is repressed by a1α2 dimer (Hongay et al., 2006).
Additionally, the promoter of SLZ1 is strictly regulated by Ume6 and Ime1, which are the key transcription factors that control early meiotic gene transcription (EMGs) (Schwartz et al., 2013) (Figure 3b).
Outside meiosis, Ume6 interacts with Sin3-Rpd3L to repress the promoters of EMGs including SLZ1, however, when MATa/α cells are starved Ime1 is expressed and activates SLZ1 transcription (Washburn & Esposito, 2001).Hence, Slz1 is only expressed during early meiosis.Both SLZ1 and IME4 transcript levels decline as cells progress into meiotic divisions, which likely explains why m6A decreases later in the sporulation programme (Agarwala et al., 2012).
While the composition of the yeast MTC has been characterised, little is known about where m6A is deposited and the molecular function of m6A during yeast meiosis.In mammals, m6A is deposited during transcription.Recent work showed that splicing sites prevent m6A deposition, which explains 3′-end bias of m6A sites in mammalian mRNAs (Zhou et al., 2019).In yeast, only approximately 150 genes show RNA splicing activity, hence a different mechanism must be in place for yeast to restrict m6A deposition.Some of the yeast MTC subunits display striking localisation patterns in cells, which may provide insights into where m6A deposition could take place.First, Slz1 has been described to be enriched in the nucleolus which has been proposed to contribute to m6A deposition (Schwartz et al., 2013).Second, the Kar4 subunit of the MTC can, together with Ste12, act as a transcription factor important for the mating response and localises to nucleus during mating.However, during meiosis Kar4 has no clear localisation pattern and does not stably associate with chromatin (Ensinck et al., 2023;Lahav et al., 2007).It is worth noting that m6A-modified mRNAs are significantly enriched with polyribosomes compared to total RNA fraction suggesting that m6A-modified mRNAs are stable at least until they associate with the translation machinery (Bodi et al., 2015).Thus, in yeast the mechanism by which specific mRNA regions are m6A-modified by the MTC likely differs from those described in mammals and possibly does not involve transcription and chromatin.
One way to gain insight into the function of the m6A modification is by determining the genes/pathways enriched for m6A-modified transcripts.For example, m6A serves as a mechanism to clear out mitotic transcripts during mammalian oocyte development, since the mitotic transcripts were more enriched with m6A (Hsu et al., 2017).In yeast, however, the data are less clear.Early work showed that the mRNA of IME2, a key meiotic kinase, is m6Amodified, which was confirmed by several other studies (Bodi et al., 2010;Schwartz et al., 2013;Varier et al., 2022).Transcriptome-wide m6A-seq analysis showed that meiotic genes were not specifically enriched among the 1000s of m6A-modified mRNAs (Dierks et al., 2021;Schwartz et al., 2013).However, another study suggested that mRNAs important for meiosis were abundant among m6A-modified mRNAs (Varier et al., 2022).Despite these discrepancies, several key genes important for meiosis (e.g., IME1 and IME2) and specifically for meiotic prophase (e.g., HOP2 and SAE2) are m6A-modified during early meiosis (Bodi et al., 2010;Schwartz et al., 2013;Varier et al., 2022).Given the importance of these meiotic transcripts for progressing into meiosis, it seems less plausible that the primary function of m6A is to stimulate mRNA turnover as suggested in mammals.
The regulation of RME1 during meiotic entry in the S288C strain background is particularly intriguing (Bushkin et al., 2019;Moretto et al., 2018).In contrast to the SK1 strain background, in S288C RME1 is expressed due to a single-nucleotide polymorphism (SNP) in the a1α2 site in its promoter (Deutschbauer & Davis, 2005).The cells harbouring a mutation in RME1 that prevents m6A display a reduction in premeiotic DNA replication (Bushkin et al., 2019).
Notably, there is another feedback loop that suppresses the inhibitory role of Rme1 on meiotic entry.This involves the transcription of a second noncoding RNA, IRT2, upstream within the IME1 promoter which represses Rme1 binding to the IME1 promoter (Moretto et al., 2018;Moretto et al., 2021;van Werven et al., 2012).

| YTH DOMAIN CONTAINING PROTEIN, Mrb1/Pho92, IS A CONSERVED READER PROTEIN
The fate of m6A-modified mRNA is determined through proteins that specifically associate with the m6A modification, also known as m6A reader proteins.The first identified and most well studied m6A readers are YTH domain-containing family of proteins (Chen et al., 2023;Patil et al., 2018;Shao et al., 2021).Over the years YTH domain-containing proteins have been assigned molecular functions in controlling decay and/or translation of m6A-modified transcripts.
In higher eukaryotes, multiple different YTH domain-containing proteins exist (e.g., five in humans, and 13 in the model plant Arabidopsis thaliana) often with redundant roles, making it challenging to decipher their molecular functions (Amara et al., 2023;Xu et al., 2021).In yeast, however, PHO92/MRB1 encodes for the only YTH domain-containing protein and is the only known m6A reader protein in yeast identified to date (Schwartz et al., 2013;Varier et al., 2022).
The YTH domains of Pho92 and the human YTHDF2 show high sequence and structural similarities and have been suggested to be orthologues of each other (Table 1) (Xu et al., 2015).
Similar to SLZ1, the promoter of PHO92 is under tight control of the EMG programme indicating that the primary role of Pho92 is restricted to early meiosis which is also the time when m6A-modified transcripts are most abundant (Scutenaire et al., 2023;Varier et al., 2022) (Figure 3b).However, Pho92 was initially identified as regulator of phosphate metabolism by controlling the stability of PHO4 transcript (Kang et al., 2014).Genome-wide mapping of Pho92-mRNA interactions using individual-nucleotide resolution CLIP (iCLIP) revealed that Pho92 associates with mRNAs in a m6Adependent and m6A-independent manners (Varier et al., 2022).
Other YTH reader proteins have been reported to be binding in an m6A-independent manner.For example, in fission yeast the YTH domain of Mmi1 has no affinity to m6A, while unstructured amino terminal domain of Mmi1 is required for the interaction of Mmi1 with Ccr4-Not complex (Stowell et al., 2016).Importantly, transcripts where Pho92 associates in an m6A-dependent manner were functionally enriched for genes important for meiosis, indicating that Pho92 directly controls the fate of meiotic transcripts (Varier et al., 2022).
What is the function of Pho92 in m6A regulation of gene expression?Cells harbouring a gene deletion in PHO92 display increased abundance of m6A-modified transcripts (Scutenaire et al., 2023;Varier et al., 2022).Moreover, there is a Pho92-dependent effect on mRNA stability where m6A-modified transcripts are less stable than unmodified transcripts, indicating that Pho92 promotes the decay of its targets (Varier et al., 2022).Consistent with these findings, Pho92 decreases the transcript abundance of a subset of EMGs during the transition from prophase into the meiotic divisions (Scutenaire et al., 2023).Since pho92Δ cells display a delay in meiotic progression, it remains to be determined whether the delay in repression of EMG expression observed in pho92Δ cells is a direct effect (Varier et al., 2022) (Table 1).
The role of Pho92 in budding yeast differs significantly from that of Mmi1 in fission yeast.Mmi1 prevents untimely entry of cells into meiosis by anchoring meiotic transcripts to nuclear foci and markedly decreasing their stability in mitotic cells, and this function is m6Aindependent (Harigaya et al., 2006;Shichino et al., 2018).The m6Aindependent function of Mmi1 is largely attributed to the radical differences in the structure of the aromatic cage domain between Mmi1 and its counterparts in budding yeast and higher eukaryotes (Wang et al., 2016).
Mechanistically, there is evidence that the decay of m6A-modified transcripts bound by Pho92 depends on translation and the Ccr4-Not complex, the major deadenylation complex in eukaryotic cells.In fission yeast and mammals, Ccr4-Not interacts directly with Mmi1 and YTHDF2, respectively.This interaction has been demonstrated to facilitate the degradation of m6A-modified transcripts, indicating that the process by which Pho92 promotes the decay of m6A-modified transcripts is conserved (Du et al., 2016;Stowell et al., 2016;Ukleja et al., 2016).An interaction between Pho92 and Pop2 of the Ccr4-Not complex has been reported, however, the specific mechanism by which Pho92 recruits Ccr4-Not is yet to be determined (Kang et al., 2014).In addition to its deadenylation function, Ccr4-Not plays key roles in regulating translation (Cooke et al., 2010;Ozgur et al., 2015).
For example, Ccr4-Not complex has been proposed to directly regulate codon optimality by promoting decay of transcripts with low codon optimality, and also its functions in regulating translation elongation have been described (Buschauer et al., 2020).There is evidence that Pho92 also contributes to translation of its targets.
Pho92 bound transcript showed overall higher translation efficiency and Pho92 cosediments with polyribosome bound mRNAs (Varier et al., 2022) (Figure 4).Perhaps Pho92 promotes both decay and translation of m6A-modified transcripts and Ccr4-Not mediates this dual role of Pho92 (Varier et al., 2022).Elucidating the molecular function of Pho92 may be key to understanding the function of m6A in yeast and could provide important insights on how m6A regulates gene expression in general.

| FUNCTIONS OF m6A OUTSIDE MEIOSIS
While yeast meiosis is when both m6A and Pho92 are most abundant, there are a few studies suggesting potential roles of the m6A mark in other phases of the yeast life cycle (Tardu et al., 2019).
For example, biochemical detection of mRNA modifications in haploid cells revealed that the m6A modification was detected among the six high-confidence mRNA modifications (Tardu et al., 2019).The m6A-modified mRNAs detected in haploid cells code for functionally related proteins that are involved in glycolysis, acetyl-CoA synthesis and shikimate/aromatic amino acid synthesis (Zhu et al., 2023).Additionally, ime4Δ cells display significant increase in the transcript level of the long-chain fatty acid acyl-CoA synthetase gene (FAA1) which leads to increase in total lipids thereby affecting vacuole morphology (Yadav & Rajasekharan, 2017).In mammalian cells there is evidence that m6A controls autophagy, suggesting that a role in protein/organelle recycling could be conserved from yeast to humans (Hao et al., 2022;Tang et al., 2021).Overexpression of Ime4 leads to small increase in m6A levels in haploid cells (Zhu et al., 2023).
Despite the repression of IME4 by antisense transcription, these findings shed the light on a possible role of m6A-modified mRNAs in haploid cells.However, the impact of the m6A mark on the fate of the mRNA in haploid cells remains to be determined.The molecular mechanisms by which m6A controls gene expression are currently not well understood.A conserved role in decay through the Pho92 reader protein is supported by the recent studies (Scutenaire et al., 2023;Varier et al., 2022).
However, given that m6A-modified transcripts and Pho92 associate with polysomes, a potential role in translation should not be excluded.If true, m6A-modified mRNAs would be prioritised over unmodified ones for the translation-to-decay fate.It is possible that instead of fine-tuning gene expression, the m6A mark serves as an mRNA prioritisation system where modified mRNAs are given preference for translation and subsequent decay over unmodified mRNAs.
Another exciting direction is to dissect the molecular mechanism by which MTC controls gene expression.One key outstanding question is about the specificity of the yeast MTC.Apart from the GGAC motif, it is not yet known in yeast why some mRNAs are m6Amodified while others are not.A second key question relates to a noncanonical function of the yeast MTC.While the primary function of the yeast MTC is to deposit m6A on mRNAs, the yeast MTC also has an important noncatalytic function acting on mRNAs (Figure 2b).
Interestingly, the noncatalytic function of the MTC is of greater importance for progression into meiosis than the catalytic function (Ensinck et al., 2023).
Future work on the molecular mechanisms underlying the roles of the yeast MTC and Pho92 in mRNA decay and translation will serve as a pivotal step in comprehending how m6A modulates gene expression not only in yeast meiosis but also reaching broader domains, including the animal and plant kingdoms.
Phenotypes in m6A levels and progression into meiosis for the mutants in MTC components and the m6A reader PHO92 in the sporulation proficient SK1 strain background.The phenotypes for catalytic inactive mutant of IME4 (IME4 CD ) and a point mutation in the YTH domain disrupting m6A binding in PHO92 (PHO92 W177A ) are also indicated (Varier et al., 2022).
First, the yeast MTC consists of a core comprised of Ime4, Mum2 and Vir1.Biochemical analysis showed that Kar4 is essential for stabilising the MTC core on mRNAs, which exerts an m6A-independent function during meiosis(Ensinck et al., 2023) (Figure2b).Slz1 is largely required for the MTC's catalytic activity, but not for stability of the MTC(Ensinck et al., 2023) (Figure2b).Slz1 also has been suggested to direct the localisation of the MTC to the nucleolus(Schwartz et al., 2013).The human orthologue of Slz1, ZC3H13, play analogous roles in the MTCs activity and the localisation of the MTC(Su et al., 2022;Wen et al., 2018).5 | REGULATION OF m6A BY THE YEAST MTCThe MTC dynamically regulates m6A deposition during the yeast sporulation programme(Schwartz et al., 2013).Accumulation of the m6A mark takes place specifically upon entry of cells into the sporulation programme and peaks in meiotic prophase(Agarwala et al., 2012;Schwartz et al., 2013).The m6A mark then starts to gradually decline as cells progress further into meiosis.The dynamic regulation of m6A in yeast contrasts that of mammals and plants where m6A is thought to be present in all cell types(Liu et al., 2022;Shao et al., 2021).How is m6A dynamically regulated in yeast?First, IME4 transcription is repressed by antisense transcription (asIME4) in yeast with a single mating type (MATa or MATα) (Figure3a).IME4 is expressed in MATa/α cells (which are typically diploid) when asIME4 Composition of the mammalian and yeast methyltransferase complex (MTC).(a) Composition of mammalian and yeast MTC.The colour matching subunits are orthologs of each other.The mammalian MTC has the MAC and MACOM composition, while the yeast MTC is likely a single complex (Ensinck et al., 2023).(b) Composition of the MTC required for m6A deposition (m6Adependent) and for m6A-independent functions.(a) (b) F I G U R E 3 Transcriptional regulation of the yeast MTC and m6A reader.(a) IME4 transcription is repressed by antisense transcription (asIME4) in cells with a single mating type, MATa or MATα (typically haploid cells).Cells harbouring both mating type, MATa and MATα (typically diploid cells) asIME4 is repressed by a1α2 heterodimer complex.Hence, IME4 is expressed in MATa/α cells.(b) Both SLZ1 (MTC component) and PHO92 (m6A reader) are under strict control of the early meiotic transcription factors Ime1 and Ume6.In MATa/α cells induced to enter meiosis, Ime1 and Ume6 will interact to drive the transcription of SLZ1 and PHO92.Under conditions other than early meiosis, SLZ1 and PHO92 transcription is repressed.
transcription factor Rme1 represses IME1 by promoting transcription of a long noncoding RNA called IRT1 through the IME1 promoter (van Werven et al., 2012).The m6A modification in RME1 promotes the decay of the RME1 mRNA, allowing for an increase in IME1 transcription and thus facilitating progression into meiosis.S288C

F I U E 4
Schematic of Pho92 function in decay and translation of m6A mRNAs.Pho92 is associated with polyribosomes where it likely is important for mRNA decay of m6Amodified transcripts.The decay depends on translation and the Ccr4-Not complex.Pho92 function has also been associated with translation.Tight control of gene expression is particularly important when transitioning from cell fate to another.It is therefore not surprising that there are various gene expression mechanisms set in place to facilitate this process.By modifying a large subset of mRNAs with the m6A mark, it is possible to fine-tune gene expression.As such the m6A modification in yeast is potentially a fine tuner of gene expression as has been proposed in other biological systems, but perhaps there is more to uncover.
Phenotypes in m6A status and meiosis for mutants in MTC components and m6A reader.