We compared the distribution of mutations in rpoB that lead to rifampin resistance in strains with differing levels of polymerase IV (Pol IV), including strains with deletions of the Pol IV-encoding dinB gene, strains with a chromosomal copy of dinB, strains with the F128 plasmid, and strains with plasmid amplification of either the dinB operon (dinB-yafNOP) or the dinB gene alone. This analysis identifies several hot spots specific to Pol IV which are virtually absent from the normal spontaneous spectrum, indicating that Pol IV does not contribute significantly to mutations occurring during exponential growth in liquid culture.Damage-inducible polymerases (20, 22; for reviews, see references 9 and 16), such as the SOS-induced polymerase IV (Pol IV) and Pol V in Escherichia coli, not only bypass certain noncoding lesions but also increase replication errors across from normal bases (19,20,23). Their discovery has led to the suggestion that a significant fraction of spontaneous mutations in growing cells under normal conditions might be due to errors caused by basal levels of error-prone polymerases (18). The dinB-encoded Pol IV is the leading candidate, since the overexpression of dinB on high-copy plasmids leads to increases in base substitutions and frameshifts, particularly Ϫ1 frameshifts (11,12,23). Moreover, several studies have shown an approximately twofold decrease in spontaneous mutations in strains with an inactivating allele of dinB that also reduces the expression of three genes downstream of dinB-yafNOP (14, 18), although this effect is not present if only dinB is inactivated (14). The expression of dinB and yafNOP is increased after SOS induction by DNA-damaging agents (4), and these four genes have been shown to be part of an operon (14).We decided to examine the spectra of base substitution mutations in strains with differing levels of dinB expression, since a comparison of detailed genetic fingerprints of these strains might reveal patterns specific to processes involving and not involving Pol IV. We recently characterized a system using mutations in the rpoB gene that yield the rifampin resistance (Rif r ) phenotype at 37°C in order to analyze the base substitution profiles of mutagens and mutators (10). We have now characterized 77 mutations in rpoB. Each of the six base substitutions is monitored with a set of 9 to 17 sites. In the study reported here, we looked at cells that carry a single copy of the dinB operon on the chromosome and compared the mutational spectrum of these cells with those of strains with deletions of the dinB gene, strains that carry a second copy of the dinB operon on an FЈ plasmid, and strains that carry a multicopy plasmid with an insert containing the dinB operon in one case and just the dinB gene in another case. We showed that some mutational hot spots are specific for the overexpression of the dinB operon and that others are found in the spectrum of wild-type strains but not after the amplification of the dinB operon. A comparison of the different spectra leads us...
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