Structure and Expression of Prokaryotic tRNA Genes

This chapter presents a summary of information on the structures and organization of prokaryotic tRNA genes based on recent work surveying the total tRNA populations and tRNA genes in Escherichia coli by Ozeki's group at Kyoto and in Mycoplasma capricolum by Osawa's group at Nagoya. Most importantly, E. coli and M. capricolum are the only cases to date in which the total tRNA systems have been well elucidated, including the sequences of a complete set of tRNAs, the organization and expression of their genes, composition of the full anticodon set, and the relative levels of each tRNA species in the cells. In general, tRNA genes are found in clusters. For example, among 79 tRNA genes of E. coli, only 20 exist as a single gene. The remaining 59 are organized in polycistronic operons containing identical or unrelated tRNA, rRNA genes, and protein-encoding genes. In M. capricolum, 22 of 30 tRNA genes are organized in five clusters comprising nine, five, four, and two genes, respectively. The other eight genes exist as single transcription units. In summary, the decoding system of M. capricolum is significantly different from that of E. coli in the number of anticodons, codon recognition patterns, and, in two cases, codon assignment.