Spontaneous mutagenesis is elevated in protease-defective cells |
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Authors: | Abu Amar M Al Mamun M Zafri Humayun |
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Institution: | University of Medicine and Dentistry of New Jersey –New Jersey Medical School, Department of Microbiology and Molecular Genetics, International Center for Public Health, Newark, NJ 07101, USA. |
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Abstract: | As a first step towards describing the role of proteolysis in maintaining genomic integrity, we have determined the effect of the loss of ClpXP, a major energy-dependent cytoplasmic protease that degrades truncated proteins as well as a number of regulatory proteins, on spontaneous mutagenesis. In a rifampicin-sensitive to rifampicin-resistance assay that detects base substitution mutations in the essential rpoB gene, there is a modest, but appreciable increase in mutagenesis in Δ( clpP-clpX ) cells relative to wild-type cells. A colony papillation analysis using a set of lacZ strains revealed that genetic −1 frameshift mutations are strongly elevated in Clp-defective cells. A quantitative analysis using a valine-sensitive to valine-resistance assay that detects frameshift mutations showed that mutagenesis is elevated 50-fold in Clp-defective cells. Elevated frameshift mutagenesis observed in Clp-deficient cells is essentially abolished in lexA1 Ind-] (SOS-uninducible) cells, and in cells deleted for the SOS gene dinB , which codes for DNA polymerase IV. In contrast, mutagenesis is unaffected or stimulated in cells deleted for umuC or umuD , which code for critical components of DNA polymerase V. Loss of rpoS , which codes for a stress-response sigma factor known to upregulate dinB expression in stationary phase, does not affect mutagenesis. We propose that elevated DinB expression, as well as stabilization of UmuD/UmuD' heterodimers in Δ( clpP-clpX ) cells, contributes to elevated mutagenesis. These findings suggest that in normal cells, Clp-mediated proteolysis plays an important role in preventing gratuitous mutagenesis. |
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