Protein synthesis is initiated by a multistep process involving disassembled ribosomal subunits (30S and 50S), initiation factors (IFs, i.e. IF1, IF2, and IF3), the initiator tRNA (fMet-tRNAfMet) and mRNAs (Laursen et al., 2005; Malys & McCarthy, 2011). For a canonical mRNA, that is, an mRNA carrying a 5′UTR, the first step consists of the formation of a binary complex made up by the 30S ribosomal subunit associated with the RBS of the mRNA, where the SD sequence base pairs with the anti-SD sequence at the 3′ tail of the 16S rRNA gene. In a second step, the 50S ribosomal subunit associates with the complex 30S mRNA, resulting in the 70S translation initiation complex. Besides this ‘classical’ view, a substantial number of leaderless mRNAs (lmRNAs) efficiently translated have been discovered in the three domains of life (Moll et al., 2002; Laursen et al., 2005; Malys & McCarthy, 2011). The translation initiation process is not yet totally elucidated, but lmRNAs are preferentially bound by 70S ribosomes without prior disassembly compared with the 30S subunit, and once formed, the 70S–lmRNA initiation complex is able to engage the elongation process without requiring IFs (Udagawa et al., 2004; Brock et al., 2008). Although certainly not dominant in bacterial species, in silico predictions and primary transcriptomes show that at least 15% of the genes would be transcribed as leaderless in certain genera; for example, Actinobacteria, Helicobacter, Xanthomonas, Deinococcus (Sharma et al., 2010; Zheng et al., 2011; Schmidtke et al., 2012). The significance of the coexistence of mRNAs and lmRNAs is currently not understood, but under certain circumstances, it may be used to respond to environmental changes. In E. coli, in response to environmental conditions triggering the toxic effect of MazF, an endoribonuclease belonging to the mazEF TA module family, certain mRNAs are specifically processed into lmRNAs and are selectively translated by specialized ribosomes (Amitai et al., 2009; Vesper et al., 2011). The MazF induction shuts off synthesis of most of proteins, and only 10% are specifically produced, including proteins provoking death in the majority of cells within a population and survival in a subpopulation (Amitai et al., 2009). MazF cleaves ssRNA at specific sequences (5′-ACA-3′) located very close upstream of the translation initiation codon, converting mRNAs with 5′ UTRs into lmRNAs (Fig. 5) (Zhang et al., 2003; Vesper et al., 2011). Within the bulk of RNAs targeted by MazF, the 16S rRNA is also processed in 30S subunits and 70S ribosomes. MazF cleaves at the 3′ terminus of the rRNA and removes the anti-SD sequence. This action of the endoribonuclease MazF generates a subpopulation of ‘stress ribosomes’ that no longer recognize mRNAs but instead specifically translate lmRNAs (Vesper et al., 2011). Thus, the MazF-mediated maturation, that switches RNAs to lmRNAs and 70S ribosomes to stress ribosomes, appears as a RNA maturation process dedicated to the translation of specific proteins in response to harmful conditions.