Common evolutionary origin of the ilvGMEDA attenuation locus and tRNA(1Leu) in Escherichia coli

Abstract

Published sequences of transcripts from ilvGMEDA leader regions of several enteric bacteria were compared with published sequences of the tRNAs from Escherichia coli. The analyses revealed homology between the ilvGMEDA leader peptide-coding region and tRNALu in E. coli, Salmonella typhimurium, and Klebsiella aerogenes, whereas homology was not present in Serratia marcescens and Edwardsiella tarda.An attenuation mechanism has been proposed to account for multivalent control of transcription of the ilvGMEDA control group in Escherichia coli (12,14). Transcription starts for ilvGMEDA are postulated to be halted by the formation of a terminator structure in the transcript of the leader region (12,14). Shortages of valyl-, isoleucyl-, or leucyl-tRNA are postulated to cause ribosomes to stall at three Val, three Ile, or two Leu codons in the leader mRNA transcript and to cause an antiterminator structure to form to allow transcription into the downstream structural genes. The regulatory region for ilvGMEDA in E. coli is remarkably similar to the ilvG control regions in other enteric bacteria such as Salmonella typhimurium and Klebsiella aerogenes but differs from that of Edwardsiella tarda and Serratia marcescens (7,20). A subject of considerable interest is how regulatory sequences evolve to control gene expression. Ames et al. (1) suggested that some tRNA genes and leader mRNA genes may share a common evolutionary ancestry. These investigators noted that the his operon leader region in S. typhimurium shares homology with tRNAHiS. To determine whether the leader region for ilvGMEDA shares homology with tRNA, we compared the published sequence of the ilvGMEDA leader mRNA with published sequences of tRNAs from E. coli (18,19). Our analysis revealed homology between tRNALeU and the leader peptide-coding region of the ilvGMEDA leader transcript in E. coli, S. typhimurium, and K. aerogenes. This region of the ilvGMEDA leader transcript contains the postulated secondary structure called a holo-protector (14). Nargang et al. (14) introduced the term to describe a complex secondary structure of the leader transcript that they postulated would preclude preemptor formation and thus assure transcription termination at the attenuator when transcription and translation are uncoupled.The microcomputer programs FASHOM and ALIGNSEQ (2, 6) were used to align the antisense strand of the ilvGMEDA leader region with the antisense strands of tRNA-coding regions (18,19). The algorithm of Needleman and Wunsch (15) was used to increase the number of base matches. Similarity scores resulting from the alignments were assessed by the method of Smith et al. (17) and expressed relative to the mean best similarity score. The mean best similarity score was calculated by the equation S = 2.55[log(nm)/log(1/P)] -8.99(1) where S is the mean best similarity, n and m are the numbers * Corresponding author.of nucleotides in the lengths of sequences under comparison, and P = 0.25 is the probability of random matches between any two sequences composed of...

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