A genetic analysis of the complex Bacillus subtilis transcriptional apparatus is essential to understand the function, regulation, and interaction of the transcriptase components during growth and sporulation. This approach in Escherichia coli has uncovered fundamental mechanisms regulating gene expression Cole and Nomura, 1986; Lindahl and Zengel, 1986) and an analysis of the B. subtilis transcriptase will allow comoparison of the E.coli system to another bacterium that has evolved under different selective pressures. To this end we used antibody probes to isolate the alpha, beta, and beta' core subunit genes from a .lambda.gtill expression vector library. To address the question of function ans regulation of the minor sigma factors that confer promoter specifity on the polymerase core (Losick et al., 1986), we used the same approach to isolate the gene for the 37,000 dalton sigma factor, sigma-37.