INVOLVEMENT OF HELICASE-II (UVRD GENE-PRODUCT) AND DNA-POLYMERASE-I IN EXCISION MEDIATED BY THE UVRABC PROTEIN COMPLEX

The bimodal-incision nature of the reaction of UV-irradiated DNA catalyzed by the Escherichia coli uvrABC protein complex potentially leads to excision of a 12- to 13-nucleotide-long damaged fragment. The oligonucleotide fragment containing the UV-induced pyrimidine dimer is not released under nondenaturing in vitro reaction conditions. Also, the uvrABC proteins are stable bound to the incised DNA and do not turn over after the incision event. It is shown that release of the damaged fragment from the parental uvrABC-incised DNA is dependent upon either chelating conditions or the simultaneous addition of the uvrD gene product (helicase II) and the polA gene product (DNA polymerase I) when polymerization of deoxynucleoside triphosphate substrates is concomitantly catalyzed. The product of this multiprotein-catalyzed series of reactions serves as a substrate for polynucleotide ligase, resulting in the restoration of the integrity of the strands of DNA. The addition of the uvrD protein to the incised DNA-uvrABC complex also results in turnover of the uvrC protein. It is suggested that the repair processes of incision, excision, resynthesis and ligation are coordinately catalyzed by a complex of proteins in a repairosome configuration.