Fig. 36.1. Involvement of several sigma factors, which change the specificity of host RNA polymerase for transcription of phage SPO1 genes, (σ29 = σgp33 - σgp34)
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 (α
2ββ
'σ
55) 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 gp
28, gp
33 and gp
34. 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 gp
28 replaced σ
55 in the holoenzyme α
2ββ
'σ
55 and therefore gp
28 is sometimes also referred to as σ
gp28. This RNA polymerase with gp
28 can then transcribe the middle genes. The next transition involves replacement of gp
28 by gp
33 and gp
34, so that RNA polymerase may now transcribe late genes (Fig. 36.1). Thus there are atleast three sigma factors used in phages SPO1, namely σ
55, σ
gp28, σ
gp33-gp34. In tne previous topic also, we showed the replacement of σ
43 by σ
37 and σ
29 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.
Fig. 36.1. Involvement of several sigma factors, which change the specificity of host RNA polymerase for transcription of phage SPO1 genes, (σ29 = σgp33 - σgp34)