sigA is an essential gene in Mycobacterium smegmatis

sigA encodes a σ factor of the σ70 family, σA, that is found in all mycobacterial species. As σA shows high similarity to the primary σ factor in Streptomyces coelicolor, it was postulated that σA has the same role in mycobacteria. However, a point mutation in sigA, resulting in the replacement of arginine 522 by histidine, was found responsible for the attenuated virulence of the Mycobacterium bovis strain ATCC 35721. This raised the possibility that σA was an alternative σ factor specifically required for virulence gene expression. In this work, we show that sigA can not be disrupted in Mycobacterium smegmatis unless an extra copy of the gene is provided at another chromosomal site, which demonstrates that sigA is essential. To characterize the pattern of sigA expression during exponential and stationary phase in M. smegmatis, we measured the β‐galactosidase activity in a strain carrying a sigA–lacZ transcriptional fusion and monitored σA levels using Western blotting. Our results indicate that sigA is expressed throughout the growth of the culture. The essential character of sigA and its pattern of expression corroborate the hypothesis that sigA codes for the primary σ factor in M. smegmatis and, most likely, in all mycobacteria.