During spermatogenesis, germ cells are first mitotically amplified, then they enter the meiotic prophase, pass through meiosis, and assemble the highly complex sperm within several days (for a review, see Fuller,1993). In postmeiotic stages, the nucleus is shaped to a needle-like form, which is accompanied by chromosome condensation. In a complex morphogenetic process, the mitochondria fuse and form the nebenkern, which then unravels and elongates to the major and minor mitochondrial derivative along the developing axoneme.
During sperm individualization, the male germline cyst is separated into single gametes by packaging each spermatid into its own plasma membrane. A cytoskeletal membrane complex assembles at the nuclear end of the cyst, which proceeds along the cyst as the so-called individualization complex. The region in which individualization occurs is enlarged and called cystic bulge (Tokuyasu et al.,1972).
It is a unique and characteristic feature that the postmeiotic stages lack transcription nearly completely in Drosophila, while in mammals a short postmeiotic transcriptional phase exists (for review see Fuller,1993, Schäfer et al.,1995, Renkawitz-Pohl et al.,2005). During the primary spermatocyte growth phase of meiotic prophase, the cell size increases and the nucleus is highly transcriptional active (Olivieri and Olivieri,1965). During this time, many mRNAs are synthesized that are translationally repressed until the appropriate time during sperm morphogenesis when the encoded protein is required for spermatid differentiation. The transcription of these translationally repressed mRNAs, for example dj mRNA (Santel et al.,1998), underlies a distinct control, as they are dependent on the tissue specific dTAFII80 homologue Can in the transcription initiation complex (White-Cooper et al.,1998; Hiller et al.,2001). The analysis has been pioneered by the identification of a translational control element (TCE) of 12 nucleotides in the 5′ untranslated region in the Mst(3)CGP gene family (Kuhn et al.,1988; Schäfer et al.,1990; Kempe et al.,1993). The tissue-specific transcripts of the dhod and the janus B gene contain an element with sequence similarity to the TCE of the Mst(3)CGP gene family (Yanicostas and Lepesant,1990; Yang et al.,1995). During sperm morphogenesis, the required proteins have to be synthesized in a coordinated timing. Therefore, it might be expected that individual mRNAs contain distinct translational repression elements. DJ indeed is synthesized earlier than the proteins and the functional analysis of the dj mRNA revealed a distinct translational repression element, TRE, within a stem and loop structure shortly before the translational initiation codon (Blümer et al.,2002). In addition to the cis regulatory elements, interacting proteins are required that bind to these sequences and keep them in a complex inaccessible to polysomes. Analyses of several RNA-binding proteins, such as Arrest, Tsr, Boule, and Rb97D, revealed that they are required for male fertility (for review, see Renkawitz-Pohl et al.,2005). So far, Boule is the only RNA binding protein for which a target mRNA, twine, is known (Eberhardt et al.,1996). Otherwise, there is no direct correlation between the characterized cis regulatory elements, TCE and TRE, and these RNA-binding proteins.
Here we characterize the expression of djl mRNA, which is transcribed in primary spermatocytes and translated with several days' delay in the elongated spermatid stage. Myc-DJL localizes to the sperm head during chromatin condensation and accumulates in the elongated flagellum but is not present in the mature sperm. We show that the DJL protein is 42% identical to DJ, which might suggest functional redundancy. Both proteins have a strongly overlapping expression pattern and their transcription depends on the tissue-specific TAFII80 Can. Their transcripts, however, are translationally controlled by distinct translational repression elements.