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Lane contained approximately pg of protein and lane M, molecular weight markers. The sedimen tation coefficient and gel band location are consistent with repair factor being a monomeric form of DNA polymerase I with a molecular weight of approximately. This may explain the unusual profile of repair activity upon hydroxylapatite with respect to polymerase activity. The large and highly active peak of a polymerase, situated over the trailing edge of the polymerase activity peak, likely contributesto the apparent stimulation of repair activity in those trailing fractions.Panels C and D show the effect of control monoclonal P antibody <a href="http://www.targetmol.com/compound/Dyclonine-hydrochloride"></a> onthesame two fractions.A form of DNA polymerase that hassignificant activity on natural DNA substrates as well aspoly and from rabbit bone marrow. While this may suggest a role for theexonuclease in DNA repair, an alternative interpretation could be th at mM AMP affects nucleotide pools or interferes with some other necessary ac tivity in thepermeabilized cells.This effect by T endonuclease V with normal cells has been previously reported. Thepreferred template is poly, butsignificant activity was also observed using activated salmon sperm DNA.DNA polymerase was unableto utilize poly, thusdistinguishing this polymerase from DNA polymerasey. Thatthis DNA repair synthesis requires DNA polymerase is based on thefollowing evidence.It sedimented through glycerol gradients and migrated through polyacrylamide gels atrates comparable to those for a poly peptide of, daltons.This factor copurified with the two activities characteristic of DNA polymerase: namely, DNA polymerase activity with poly and to exonuclease activity;it is clearly separable from other activities knownto be implicated in DNA repair.This conclu sion is also supported by recent reports th atconcluded th at DNA replication and repair may be atleast partly dependent upon DNA polymerase. In fact, while inactive in mediating DNA repair alone, apolymerase increases DNA repair synthesis by when added with polymerase, suggesting th atapolymerase might be able to catalyze some repair synthesis subsequent to or in conjunction with the reaction of polymerase.Thus, at least one function for DNA polymerase appears to exist th at is distinct from th atof DNA polymerase a.The most distinctive feature is theassociation of the polym erase with a   to exonuclease, and like the nucleases of all previously isolated polymerases, this nuclease is inhibited by AMP. The potential for an in uitro complementation assay to be able to identify mammalian DNA repair factors has engen dered the development of a variety of systems utilizing iso lated nuclei, cellfree extracts obtained by osmotic disruption, permeabilized cells, or viable cells microinjected with cell extracts. To date none of these has been entirely successful.We have made preliminary observations of a positive response of xe roderma pigmentosum cytosoldepleted permeabilized fibro blasts to exogenously supplied crude cell extract provided th at C. The streng th of such a system, if successful, would be its providing a means to purify such factors in anactivesta te without havingto presuppose the na ture of such activities or to design and prepare complex substrates.F, Crute, J. J, Sabatino, R. D, Bodner, J. B, Marraccino, R. L, Harwell, L. W, Lord, E. M, and Bambara, R. A. Biochemistry,

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