REGULATION OF HISTIDINE AND PROLINE DEGRADATION ENZYMES BY AMINO-ACID AVAILABILITY IN BACILLUS-SUBTILIS

The first enzymes of the histidine (hut) and proline degradative pathways, histidase and proline oxidase, could not be induced in Bacillus subtilis cells growing in glucose minimal medium containing a mixture of 16 amino acids. Addition of the 16-amino-acid mixture to induced wild-type cells growing in citrate minimal medium repressed histidine synthesis 25- to 250-fold and proline oxidase synthesis 16-fold. A strain containing a transcriptional fusion of the hut promoter to the β-galactosidase gene was isolated from a library of Tn917-lacZ transpositions. Examination of histidase and β-galactosidase expression in extracts of a hut-lacZ fusion strain grown in various media showed that induction, catabolite repression, and amino acid repression of the hut operon were mediated at the level of transcription. This result was confirmed by measurement of the steady-state level of hut RNA in cells grown in various media. Since amino acid repression was not defective in B. subtilis mutants deficient in nitrogen regulation of glutamine synthetase and catabolite repression, amino acid repression appears to be mediated by a system that functions independently of these regulatory systems.