Gene for Escherichia coli glycyl-tRNA synthetase has tandem subunit coding regions in the same reading frame

Abstract

Glycyl-tRNA synthetase is one of two Escherichia coli aminoacyl tRNA synthetases which has two different subunits. A 5.1-kilobase pair HindIII chromosomal DNA fragment was isolated, cloned into pBR322 (to give plasmid pTK201), and shown to direct synthesis in maxicells of both subunits (Mr = 35,000 (alpha) and Mr = 65,000 (beta) of glycyl-tRNA synthetase. Locations of alpha- and beta-subunit coding regions were established by introduction of Tn5 insertions into various positions within the 5.1-kilobase pair HindIII segment of pTK201 and by determining the effect of each Tn5 insertion on synthesis of alpha- and beta-subunits and on enzymatic activity. From the Tn5 insertion analysis, regions encoding the NH2 terminus of the alpha-subunit and of the beta-subunit were approximately defined and these regions were sequenced. To locate rigorously the respective NH2-terminal encoding sections in the DNA sequence, NH2-terminal amino acid sequences of alpha- and beta-subunits were established by standard Edman degradations and these sequences were aligned with the DNA sequence. This analysis established the following: 1) coding regions for the subunits are in tandem; 2) a single promoter is used for transcription of both coding sections and the order of transcription is from alpha to beta; 3) in the 500 nucleotides 5' to the start of the alpha-subunit coding section, there is no sequence arrangement like that found for regulatory regions of bacterial amino acid biosynthetic operons; 4) nine nucleotides serve as the spacer between the TAA stop of the alpha- and the ATG start of the beta-subunit coding regions, thus making both coding regions in the same reading frame; and 5) the TAA stop of the alpha-subunit and the next for nucleotides associated with the intersubunit region are complementary to the 3'-end of 16 S rRNA; this arrangement suggests ribosome re-initiation in the spacer region gives balanced synthesis of both subunits.