Regulation of expression of the Lactococcus lactis histidine operon

In Lactococcus lactis, the his operon contains all the genes necessary for histidine biosynthesis. It is transcribed from a unique promoter, localized 300 bp upstream of the first gene, The region corresponding to the untrans lated 5' end of the transcript, named the his leader region, displays the t ypical features of the T box transcriptional attenuation mechanism which is involved in the regulation of many amino acid biosynthetic operons and tRN A synthetase genes in gram-positive bacteria, Here we describe the regulati on of transcription of the his operon by the level of histidine in the grow th medium, In the absence of histidine, two transcripts are present, One co vers the entire operon, while the other stops at a terminator situated shou t 250 bp downstream of the transcription start point, DNA sequences implica ted in regulation of the his operon were identified by transcriptional fusi on with luciferase genes and site-directed mutagenesis. In addition to the previously defined sequences necessary for effective T-box-mediated regulat ion, new essential regions were identified. Eighteen percent of the positio ns of the his leader region were found to differ in seven distantly related strains of L, lactis, Analysis of the variable positions supports the fold ing model of the central part of the his leader region. Lastly, in addition to the T-box-mediated regulation, the operon is regulated at the level of initiation of transcription, which is repressed in the presence of histidin e, An operator site, necessary for full repression, overlaps the terminator involved in the T box attenuation mechanism. The functionality of the oper ator is altered on plasmids with low and high copy numbers, suggesting that supercoiling may play a role in the expression of the his operon, The exte nts of regulation at the levels of initiation and attenuation of transcript ion are 6- to 8-fold and 14-fold, respectively. Together, the two levels of control allow a 120-fold range of regulation of the L. lactis operon by hi stidine.