Identifying sigma factors in Mycobacterium smegmatis by comparative genomic analysis

Mycobacterium smegmatis is a saprophytic species that has been used for 15 years as a model to perform heterologous regulation and virulence studies of Mycobacterium tuberculosis. Members of the extracytoplasmic sigma factors family, which are required for adaptive responses to various environmental stresses, are responsible for some of the virulence traits of M. tuberculosis. A bioinformatic search on the genome of M. smegmatis has predicted the existence of 26 sigma factors, which is twice the number that are present in M. tuberculosis. A phylogenetic analysis has shown that despite this high number of sigma factors the orthologs of the genes sigC, sigI and sigK of M. tuberculosis are absent in the M. smegmatis genome. Several sigma factors are specific for M. smegmatis, with a special enrichment in the sigH and, to a lesser extent, in the sigJ and sigL subfamily, pinpointing the potential variability of the repertoire of adaptive response in this saprophytic species.