Genes specific for sporulation can be identified by spcr mutations, which will block sporulation. These mutants may be spo 0, where sporulation does not start at all; others could be spo II, etc. where later stages are affected. These genes either synthesize enzymes or structural proteins or are involved in transition from vegetative phase to sporulation.

Some of the changes which occur during change of life style from vegetative to sporulating phase are now known and have been studied to indicate that alterations in RNA polymerase are involved in the initiation of sporulation. In RNA polymerase of B. subtilis, atleast three sigma factors namely σ⁴³ (43,000 daltons), σ³⁷ (37,000 daltons) and σ²⁹ (29,000 daltons) have been identified, σ⁴³ is involved in transcription during vegetative phase, while σ³⁷, and σ²⁹ are involved during sporulation (Fig. 35.19). At the beginning of sporulation σ⁴³ is replaced by σ³⁷, perhaps due to the activity of any of the five spo 0 genes, which either removes a⁴³ or modifies the core enzyme. The displaced a⁴³ can be recovered from sporulating cells. Therefore, α₂ββ'σ³⁷ initiates the transcription of gene needed for sporulation. About four hours after sporulation, another protein σ²⁹ appears and RNA polymerase α₂ββ'σ²⁹ functions for transcription (Fig. 35.19). Several other sigma factors have been isolated and characterized in B. subtilis. Thus, it is obvious that selective expression of genes can be caused by changes in sigma factor of RNA polymerase enzyme.