Reduced expression of antigenic proteins MPB70 and MPB83 in Mycobacterium bovis BCG strains due to a start codon mutation in sigK

Mol Microbiol. 2005 Jun;56(5):1302-13. doi: 10.1111/j.1365-2958.2005.04618.x.


Mycobacterium bovis Bacille Calmette-Guerin (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, mpb70 and mpb83, 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 including mpb70 (Rv2875) and mpb83 (Rv2873) and a second that includes the predicted sigma factor, sigK. DNA sequence analysis revealed a point mutation in the start codon of sigK in all low-producing BCG strains. Complementation of a low-producing strain, BCG Pasteur, with wild-type sigK fully restored MPB70 and MPB83 production. Microarray-based analysis and confirmatory RT-PCR of the complemented strains revealed an upregulation in gene transcription limited to the sigK and the mpb83/mpb70 gene regions. These data demonstrate that a mutation of sigK is responsible for decreased expression of MPB70 and MPB83 in low-producing BCG strains and provide clues into the role of Mycobacterium tuberculosis SigK.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antigens, Bacterial / biosynthesis*
  • Bacterial Proteins / biosynthesis*
  • Codon, Initiator*
  • Gene Expression
  • Genetic Complementation Test
  • Immunoblotting
  • Membrane Proteins / biosynthesis*
  • Mycobacterium bovis / genetics*
  • Mycobacterium bovis / metabolism
  • Oligonucleotide Array Sequence Analysis
  • Point Mutation
  • RNA, Bacterial / analysis
  • RNA, Messenger / analysis
  • Reverse Transcriptase Polymerase Chain Reaction
  • Sequence Analysis, DNA
  • Transcription Factors / genetics*
  • Transcription, Genetic / genetics


  • Antigens, Bacterial
  • Bacterial Proteins
  • Codon, Initiator
  • MPB83 protein, Mycobacterium
  • Membrane Proteins
  • RNA, Bacterial
  • RNA, Messenger
  • Transcription Factors
  • sigma K
  • MPB70 protein, Mycobacterium bovis