Duplication of the structural gene for glycyl-transfer RNA synthetase in Escherichia coli

Abstract

Earlier, Folk & Berg (1970 a,b) described a class of mutants affecting the structural gene for glycyl-transfer RNA synthetase ( glyS); their Gly − phenotype (requirement for added glycine to minimal medium) was thought to result from a lowered affinity of the enzyme for glycine and consequently a reduced rate of esterification of glycine to tRNA Gly at normal intracellular levels of glycine. Such mutants revert to a Gly + phenotype at a high frequency (> 10 −5) although the glyS mutant allele remains unchanged. Our experiments show that the Gly + revertants arise because of a duplication (or perhaps an amplification) of the mutant glyS allele. The duplication can also include other genes which occur up to four minutes map distance from glyS. We surmise that the increase in the number of structural genes for glycyl-tRNA synthetase generates an elevated level of the defective enzyme in vivo thereby permitting nearly normal rates of esterification of tRNA Gly even with the otherwise limiting levels of endogenous glycine.