L-Histidine production by histidase-less regulatory mutants of Serratia marcescens constructed by transduction.

2-Methylhistidine (2MH) and 1,2,4-triazole-3-alanine (TRA) inhibited the growth of Serratia marcescens. These inhibitory effects were counteracted by L-histidine. Enzymatic studies showed that 2MH acts as a false feedback inhibitor and TRA acts as both a false feedback inhibitor and a repressor. Mutants resistant to each analog were isolated from a histidase-less mutant, because the wild-type strain possesses a potent histidase activity. 2MH-resistant mutants had a feedback-insensitive phosphoribosyltransferase, but they produced only small amounts of L-histidine. TRA-resistant mutants were divided into two types according to their histidine productivity. A mutant of one type produced about 8 mg of L-histidine per ml and had about a 10-fold increase in the enzyme levels of histidine biosynthesis. Moreover, this mutant had a partially feedback-insensitive phosphoribosyltransferase. A mutant of the second type produced only a small amount of L-histidine and had only derepressed enzyme levels. Accordingly, strains possessing the genetic alterations in both 2MH- and TRA-resistant mutants were constructed by PS20-mediated transduction. They had both feedback-insensitive phosphoribosyltransferase and derepressed enzyme levels. The representative strain HT-2604 produced about 17 mg of L-histidine per ml.