Organization and structure of an E. coli tRNA operon containing seven tRNA genes.

The structure and organization on the Escherichia coli chromosome of the gene cluster coding for two methionine tRNAs (tRNAmMet), four glutamine tRNAs (two tRNA1Gln and two tRNA2Gln), and a previously unidentified tRNA (called tRNAx) have been studied by restriction enzyme analysis and DNA sequencing, utilizing a specialized transducing bacteriophage (lambda psu degrees 2) carrying the supB-supE region. From the sequence analysis, the previously unidentified tRNA has been shown to have an anticodon sequence (5'-UAG-3') corresponding to a leucine codon. The organization of this tRNA gene cluster on the E. coli chromosome is tRNAmMet-9 base pairs-tRNAx-23 base pairs-tRNA1Gln-34 base pairs-tRNA1Gln-15 base pairs-tRNAmMet-47 base pairs-tRNA2Gln-37 base pairs-tRNA2Gln. The duplicated genes coding for tRNAmMet, tRNA,Gln, and tRNA2Gln have identical sequences, which are the same as the sequences determined previously with tRNA molecules. These tRNA sequences are preceded by a single promoter region where a "Pribnow box" sequence is present seven base pairs upstream from the transcription start site. The spacer regions separating the seven tRNA sequences are different from each other both in size and in nucleotide sequence. The possible implication of these sequences for precursor processing is discussed. A restriction fragment that has been originally identified in lambda psu degrees 2 DNA and shown to contain the seven tRNA genes has been detected in the E. coli chromosome, thereby suggesting that this tRNA gene cluster is present in the bacterial genome with the same organization as in the transducing phage genome.