Alternative sigma factors in phage SPO1

The regulation of gene expression through a cascade has been studied in phage SPO1 infecting Bacillus subtilis. It has been shown that for the expression of early genes of SPO1, bacterial RNA polymerase (α₂ββ'σ⁵⁵) is used, because the promoters of these early genes like the promoters of host genes can be recognized by this RNA polymerase holoenzyme. The consensus sequence in SPO1 at -35 and -10 base pairs resembles those of E. coli and B. subtilis. For the expression of middle and late genes, however, expression of atleast three regulator phage genes, namely 28, 33 and 34 is needed. The products of these genes are respectively called gp²⁸, gp³³ and gp³⁴. The gene 28 is an early gene, whose expression is needed for the expression of middle genes. Similarly, genes 33 and 34 are middle genes whose expression is a pre-requisite for the expression of late genes.

It has been shown that gp²⁸ replaced σ⁵⁵ in the holoenzyme α₂ββ'σ⁵⁵ and therefore gp²⁸ is sometimes also referred to as σ^gp28. This RNA polymerase with gp²⁸ can then transcribe the middle genes. The next transition involves replacement of gp²⁸ by gp³³ and gp³⁴, so that RNA polymerase may now transcribe late genes (Fig. 36.1). Thus there are atleast three sigma factors used in phages SPO1, namely σ⁵⁵, σ^gp28, σ^gp33-gp34. In tne previous topic also, we showed the replacement of σ⁴³ by σ³⁷ and σ²⁹ during bacterial sporulation, suggesting that selective transcription can be achieved by replacement of sigma factor in RNA polymerase holoenzyme. We do not clearly know about the factors which cause this replacement of one sigma factor by another, but there are different promoter sequences in different genes to be recognized by different sigma factors.