Reduced expression of antigenic proteins MPB70 and MPB83 inMycobacterium bovisBCG strains due to a start codon mutation insigK.

Mycobacterium bovisBacille Calmette–Guérin (BCG) strains are genetically and phenotypically heterogeneous. Expression of the antigenic proteins MPB70 and MPB83 is known to vary considerably across BCG strains; however, the reason for this phenotypic difference has remained unknown. By immunoblot, we separated BCG into high- and low-producing strains. By quantitative reverse transcription polymerase chain reaction (RT-PCR), we determined that transcription of the antigen-encoding genes,mpb70andmpb83, follows the same strain pattern with mRNA levels reduced over 50-fold in low-producing strains. Transcriptome comparison of the same BCG strains by DNA microarray revealed two gene regions consistently downregulated in low-producing strains compared with high-producing strains, one includingmpb70(Rv2875) andmpb83(Rv2873) and a second that includes the predicted sigma factor,sigK. DNA sequence analysis revealed a point mutation in the start codon ofsigKin all low-producing BCG strains. Complementation of a low-producing strain, BCG Pasteur, with wild-typesigKfully restored MPB70 and MPB83 production. Microarray-based analysis and confirmatory RT-PCR of the complemented strains revealed an upregulation in gene transcription limited to thesigKand thempb83/mpb70gene regions. These data demonstrate that a mutation ofsigKis responsible for decreased expression of MPB70 and MPB83 in low-producing BCG strains and provide clues into the role ofMycobacterium tuberculosisSigK. [ABSTRACT FROM AUTHOR]