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Background: The gene infB encodes the prokaryotic translation initiation factor IF2, a central macromolecular component in the formation of the ribosomal 70S initiation complex. In Escherichia coli, infB encodes three forms of IF2: IF2α, IF2β and IF2γ. The expression of IF2β and IF2γ is a tandem translation from intact infB mRNA and not merely a translation of post-transcriptionally truncated mRNA. The molecular mechanism responsible for the ribosomal recognition of the two intracistronic translation initiation sites in E. coli infB is not well characterized. Results: We found three different forms of IF2 in Enterobacter cloacae, Klebsiella oxytoca, Salmonella enterica, Salmonella typhimurium, and two different forms in Proteus vulgaris. We identified the intracistronic translation initiation sites of the mRNA by isolation and N-terminal sequencing of the shorter isoforms of IF2 in S. enterica and S. typhimurium. A further search in the readily available public sequence databases revealed that infB from Yersinia pestis also contains an intracistronic in-frame initiation site used for the translation of IF2β. The base composition in a part of the 5′ end of the DNA coding strand of the enterobacterial infB gene shows a strong preference for adenine (A) over thymine (T) with a maximum ratio of A-to-T around the intracistronic initiation sites. We demonstrate that the mRNA has an open structure around the ribosomal binding region. Conclusion: Efficient intracistronic translation initiation of the infB gene is suggested to require an mRNA with this special base composition that results in an open, single-stranded structure at the ribosomal binding region.Summary Figure Figure Summary Figure . Schematic view of the translation process of infB mRNA. The 5′-end of the infB mRNA preceding the intracistronic translation initiation codon(s) has a number of rare codons. This stalls the translation in this region of the mRNA, providing space for the ribosomes to initiate at the intracistronic initiation site(s). The region around the intracistronic sites(s) has a high content of adenine and a low content of uracil. This leads to an open single-stranded structure of the mRNA in the region, which is a necessity for the intracistronic translation initiation of the infB gene. Download figure to PowerPoint |
