The Staphylococcus aureus chromosomal gene plaC, identified by mutations amplifying plasmid pT181, encodes a sigma factor.

The Staphylococcus aureus chromosomal gene plaC, identified by mutations such as plaC1 that lead to the amplification of plasmid pT181, has been cloned and sequenced. The plaC gene encodes a protein with high similarity (79% identity) with the vegetative sigma factor of Bacillus subtilis, sigA, suggesting that it acts as an RNA polymerase sigma factor in S.aureus. The plaC1 mutation was found to be a C to T transition leading to a proline to serine substitution at amino acid residue 209 of the protein. In other sigma factors this region of the protein is involved in specific recognition of the -10 promoter sequence. The change in sigma factor activity due to this mutation is characterized by its strict specificity for a limited number of promoters and the rather high amplitude of the effect.