Free sigma subunit of Bacillus subtilis RNA polymerase binds to DNA

The affinity of Bacillus subtilis RNA polymerase sigma and delta subunits to DNA was examined by a non-denaturing polyacrylamide slab gel electrophoresis method which made it possible to resolve DNA-bound and free subunits. The results revealed that sigma subunit, but not delta subunit had a relatively high affinity for double stranded DNA. The sigma subunit was bound maximally to super-coiled pGR1-3 plasmid DNA at a mass ratio of sigma/DNA of 0.7. With B. subtilis double stranded linear DNA one sigma subunit was bound per approximately 1,000 base pairs. The sigma-DNA complex was sufficiently stable for isolation by a molecular gel filtration column. The sigma subunit had much higher affinity for super-coiled than for linear pGR1-3 DNA or for linear double stranded or denatured DNA from B. subtilis, E. coli, and calf thymus. These results indicate that the free B. subtilis sigma subunit, in contrast to the E. coli sigma subunit, can bind by itself to DNA.