STRUCTURE AND REGULATION OF EXPRESSION OF THE BACILLUS-SUBTILIS VALYL-TRANSFER-RNA SYNTHETASE GENE

We have sequenced the valyl-tRNA synthetase gene (valS) of Bacillus su btilis and found an open reading frame coding for a protein of 880 ami no acids with a molar mass of 101,749. The predicted amino acid sequen ce shares strong similarity with the valyl-tRNA synthetases from Bacil lus stearothermophilus, Lactobacillus casei, and Escherichia coli. Ext racts of B. subtilis strains overexpressing the valS gene on a plasmid have increased valyl-tRNA aminoacylation activity. Northern analysis shows that valS is cotranscribed with the folC gene (encoding folyl-po lyglutamate synthetase) lying downstream. The 300-bp 5' noncoding regi on of the gene contains the characteristic regulatory elements, T box, ''specifier codon'' (GUC), and rho-independant transcription terminat or of a gene family in gram-positive bacteria that encodes many aminoa cyl-tRNA synthetases and some amino acid biosynthetic enzymes and that is regulated by tRNA-mediated antitermination. We have shown that val S expression is induced by valine limitation and that the specificity of induction can be switched to threonine by changing the GUC (Val) sp ecifier triplet to ACC (Thr). Overexpression of valS from a recombinan t plasmid leads to autorepression of a valS-lacZ transcriptional fusio n. Like induction by valine starvation, autoregulation of valS depends on the presence of the GUC specifier codon. Disruption of the valS ge ne was not lethal, suggesting the existence of a second gene, as is th e case for both the thrS and the tyrS genes.