BACTERIAL RNA polymerase consists of a core (α2β β′) and a sigma (σ) factor which form the active holoenzyme (α2ββ′σ) (ref. 1). Significant differences in size of the β and β′ subunits occur in the enzymes from various bacterial species2. The exact functions of the larger subunits are still unknown. In Escherichia coli the β′ subunit is the largest polypeptide and is involved in the binding of RNA polymerase to DNA3; the β subunit is responsible for both rifampicin 4 and streptolydigin5,6 resistance. In contrast the largest RNA polymerase polypeptide in Bacillus subtilis is responsible for rifampicin resistance and has been designated as β (ref. 7). The second largest polypeptide of B. subtilis contains the two zinc atoms associated with RNA polymerase8 and has been designated as β′. We demonstrate here, using subunit reconstitution studies, that the second largest subunit of B. subtilis RNA polymerase - the β′ subunit - is responsible for streptolydigin resistance. Since the β subunit of E. coli is responsible for resistance to both antibiotics, a divergence in function has occurred between the β and β′ subunits of B. subtilis and E. coli. These and other studies9 thus show clearly that function cannot be attributed to the two larger subunits purely on their relative size. Furthermore the identification of streptolydigin resistance with the β′ subunit and the map locations of rifampicin and streptolydigin resistance in B. subtilis10 indicate that the genes for the β and β′ subunits are very closely linked and that streptolydigin resistance determines the rpoC locus in B. subtilis.
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