J Bacteriol, March 1998, p. 1095-1102, Vol. 180, No. 5
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
32 Determine Core RNA
Polymerase Binding Specificity

Department of Molecular and Cell Biology, University of California, Berkeley, California 94720,1 and Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 208922
Received 29 September 1997/Accepted 23 December 1997
We have analyzed the core RNA polymerase (RNAP) binding activity of
the purified products of nine defective alleles of the rpoH
gene, which encodes
32 in Escherichia coli.
All mutations studied here lie outside of the putative core RNAP
binding regions 2.1 and 2.2. Based on the estimated
Kss for the mutant sigma and core RNAP
interaction determined by in vitro transcription and by glycerol
gradient sedimentation, we have divided the mutants into three classes.
The class III mutants showed greatly decreased affinity for core RNAP,
whereas the class II mutants' effect on core RNAP interaction was only clearly seen in the presence of
70 competitor. The class
I mutant behaved nearly identically to the wild type in core RNAP
binding. Two point mutations in class III altered residues that were
distant from one another. One was found in conserved region 4.2, and
the other was in a region conserved only among heat shock sigma
factors. These data suggest that there is more than one core RNAP
binding region in
32 and that differences in contact
sites probably exist among sigma factors.
Present address: Department of Microbiology and Immunology,
University of California, San Francisco, CA 94143.
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