Amplified RNase H Activity in Escherichia coli B/r Increases Sensitivity to Ultraviolet Radiation

Richard Bockrath ¹, Lee Wolff ¹, Abigail Farr ¹, and Robert J. Crouch ²

¹ Department of Microbiology and Immunology, Indiana University School of Medicine, Indianapolis, Indiana 46223
² Laboratory of Molecular Genetics, National Institute of Child Health and Human Development, Bethesda, Maryland 20205

Strains of E. coli B/r transformed with the plasmid pSK760 were found to be sensitized to inactivation by ultraviolet radiation (UV) and to have elevated levels of RNase H activity. Strains transformed with the carrier vector pBR322 or the plasmid pSK762C derived from pSK760 but with an inactivated rnh gene were not sensitized. UV-inactivation data for strains having known defects in DNA repair and transformed with pSK760 suggested an interference by RNase H of postreplication repair: uvrA cells were strongly sensitized, wild-type and uvrA recF cells were moderately sensitized and recA cells were not sensitized; and minimal medium recovery was no longer apparent in sensitized uvrA cells. Biochemical studies showed that post-UV DNA synthesis was sensitized and that the smaller amounts of DNA synthesized after irradiation, while of normal reduced size as indicated by sedimentation position in alkaline sucrose gradients, did not shift to a larger size (more rapidly sedimenting) upon additional incubation. We suggest an excess level of RNase H interferes with reinitiation of DNA synthesis on damaged templates to disturb the normal pattern of daughter strand gaps and thereby to inhibit postreplication repair.

This article has been cited by other articles:

				E. Masse, P. Phoenix, and M. Drolet DNA Topoisomerases Regulate R-loop Formation during Transcription of the rrnB Operon in Escherichia coli J. Biol. Chem., May 9, 1997; 272(19): 12816 - 12823. [Abstract] [Full Text] [PDF]