
Full text loading...
Category: Microbial Genetics and Molecular Biology
Leaderless mRNAs in the Spotlight: Ancient but Not Outdated!, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781683670247/9781683670230_Chap10-1.gif /docserver/preview/fulltext/10.1128/9781683670247/9781683670230_Chap10-2.gifAbstract:
In bacteria and archaea, translation initiates with a 30S ribosomal subunit interacting with an initiator tRNA at the ribosome binding site on a canonical mRNA to form a stable translation initiation complex that is primed for elongation. Canonical mRNAs contain both 5′ and 3′ untranslated regions (UTRs) containing information that will influence the stability and translation efficiency of the mRNA. Within the 5′ UTR, these signals can include ribosome recognition regions such as purine-rich Shine-Dalgarno (SD) sequences that are complementary to the anti-SD (aSD) sequence near the 16S rRNA 3′ terminus ( 1 ), AU-rich sequences that interact with ribosomal protein (r-protein) bS1 ( 2 , 3 ) and prevent the formation of secondary structures, and enhancer regions. Additionally, 5′ UTRs may contain sequences that can be bound by trans-acting elements (i.e., proteins, antisense and small regulatory RNAs, or low-molecular-weight effectors) to change secondary structures or block translation initiation regions. Therefore, the regulatory and translation initiation signals are primarily contained within the 5′ UTR. Despite this functional importance of the 5′ UTR, there exists a class of mRNAs that are completely devoid of 5′ UTRs or possess very short 5′ UTRs. These mRNAs lack the SD sequence and any other translational signals and are so named leaderless mRNAs (lmRNAs). Thus, the mechanism underlying their recognition and binding by the translational apparatus is still not entirely elucidated.
Full text loading...
Mechanisms leading to the generation of lmRNAs in bacteria. Besides genes that are generally transcribed as lmRNAs, bacteria can generate lmRNAs in response to adverse environmental conditions (i) by activation of alternative promoters, where the transcriptional start point coincides with the A of the AUG start codon; (ii) by cotranscriptional cleavage, when the 5′ UTR is removed by RNases during the process of transcription; or (iii) cotranslationally. Here, the cleavage can be regulated by translating ribosomes that might either protect mRNAs from cleavage or expose specific sites for the processing event by RNases. Cleavage sites and potential RNases are indicated by red spheres and scissors, respectively.
Potential pathways for translation initiation complex (IC) formation on lmRNAs. (A) Schematic showing the main steps during canonical initiation. (B and C) Potential steps during translation initiation on lmRNAs via 30S subunits and 70S monosomes, respectively. r-Proteins bS1 and uS2 are transparent, indicating their dispensability during this process. See text for details.
Compilation of published transcriptome analyses outlining the number of leaderless mRNAs in a variety of bacterial and archaeal genomes a