Positive regulation of Bacillus subtilis sigD by C-terminal truncated LacR at translational level

DegR is a positive regulator for degradative enzyme synthesis in Bacillus subtilis. The degR gene is transcribed by RNA polymerase containing delta D, and the level of its expression is low in a mecA-deficient mutant. In a search for suppressors of the mecA effect through mini-Tn10 transposon mutagenesis, a lacR mutation designated lacR288 was discovered. The B. subtilis lacR gene encodes the repressor for lacA which specifies beta-galactosidase, and therefore, inactivation of the lacR gene results in overproduction of the enzyme. In the lacR288 mutant, however, the expression of lacA was at a negligible level, indicating that the repressor activity was not destroyed by the mutation. The putative gene product of the lacR288-containing gene is a 288-amino acid protein lacking the C-terminal 42 amino acids of intact LacR and carries no extra amino acids derived from the transposon sequence. The suppression by lacR288 of the decreased degR expression in the mecA background was found to be caused by an increase in the delta D level as shown by Western blot analysis. Furthermore, the increase was due to post-transcriptional regulation of sigD, the gene encoding delta D, as revealed by using both transcriptional and translational sigD-lacZ fusions. The lacR288 mutation had no effect on the stability of the delta D protein. Based on these results we conclude that the lacR288 mutation stimulates sigD expression at the translational level.