Overproduction and purification of a bacteriophage SPO1-encoded RNA polymerase sigma factor

Gene 28 of bacteriophage SPO1 encodes an RNA polymerase sigma factor sigma gp28, which replaces the host Bacillus subtilis sigma subunit sigma 55, to alter the promoter recognition specificity of RNA polymerase. A fragment of SPO1 DNA containing gene 28 was placed under the control of the PL promoter of bacteriophage lambda in an Escherichia coli expression vector. When transcription of gene 28 was induced by derepression of PL, the sigma gp28 synthesized constituted several per cent of total cellular protein. Sigma gp28 purified from these cells was able to confer specificity for SPO1 middle gene promoters upon B. subtilis core RNA polymerase, and also enabled E. coli core RNA polymerase to recognize and initiate transcription from an SPO1 middle gene promoter.