Structure and organization of the two tRNA Tyr gene clusters on the E. coli chromosome

Abstract

The structure and organization of the gene clusters coding for the two tyrosine-accepting tRNA species (tRNA 1 Tyr and tRNA 2 Tyr) on the E. coli chromosome have been determined. The mature structural sequences of the two tRNA Tyr genes, located on opposite sides of the E. coli chromosome, differ by only 2 bp, but sequences surrounding these portions of the genes are very different. The genes coding for tRNA 1 Tyr ( tyrT) comprise two mature structural sequences separated by a 200 bp “intergenic spacer.” It is known that in transducing phage, the region adjoining the CCA end of the second mature structural sequence comprises a 178 bp repeated sequence which contains an in vitro, rho-dependent transcriptional termination site. We find that these potentially genetically unstable repeated sequences are present in the E. coli chromosome with the same organization as that determined from transducing phage analyses. The gene that codes for tRNA 2 Tyr ( tyrU) is present in a single copy and is tightly clustered with three other tRNA genes. One of these genes (to be called thrU) encodes a previously undescribed tRNA (to be called tRNA 4 Thr). The organization of this cluster on the E. coli chromosome is tRNA 4 Thr − 8 bp − tRNA 2 Tyr − 115 bp − tRNA 2 Gly − 6 bp − tRNA 3 Thr. The importance of correlating structural analyses derived from specialized transducing phage with those determined for the chromosome itself is demonstrated by results which show that out of four independently isolated tRNA Tyr transducing phage, two carrying the tRNA 1 Tyr genes [φ80psu 3 +,− (Cambridge) and φ80sus 2psu 3 + (Kyoto)] and two carrying the tRNA 2 Tyr gene (λ rif d 18 and λh80d glyTsu 36 +), only the first phage from each group has the same gene organization as that found in the E. coli chromosome