SIGA IS AN ESSENTIAL GENE IN MYCOBACTERIUM-SMEGMATIS

sigA encodes a sigma factor of the sigma(70) family, sigma(A), that is found in all mycobacterial species. As sigma(A) shows high similarity to the primary sigma factor in Streptomyces coelicolor, it was postul ated that sigma(A) has the same role in mycobacteria. However, a point mutation in sigA, resulting in the replacement of arginine 522 by his tidine, was found responsible for the attenuated virulence of the Myco bacterium bovis strain ATCC 35721. This raised the possibility that si gma(A) was an alternative a 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 provid ed at another chromosomal site, which demonstrates that sigA is essent ial. To characterize the pattern of sigA expression during exponential and stationary phase in M,smegmatis, we measured the beta-galactosida se activity in a strain carrying a sigA-lacZ transcriptional fusion an d monitored sigma(A) levels using Western blotting. Our results indica te that sigA is expressed throughout the growth of the culture. The es sential character of sigA and its pattern of expression corroborate th e hypothesis that sigA codes for the primary sigma factor in M. smegma tis and, most likely, in all mycobacteria.