P h u l~~~h i~m i~l r~o t i~l P l m r u b k~l u~? .Abstract -The action of an endonuclease from Micrococcus Iuieus. that operates on ultraviolet ( U V ) radiation damage, overlaps greatly with that of the yeast photoreactivating enzyme: homo and hetero cyclobutyl pyrimidine dimers in DNA are substrate for both enzymes, but pyrimidine adducts or the 'spore photoproduct' in DNA are not.As expected from this overlap, the action of the two enzymes is mutually interfering: single-strand nicks introduced by the endonuclease effectively preclude photoreactivation: conversely. formation of a photoreactivating enzyme-dimer complex can prevent nicking by the UV endonuclease. While complex formation between photoreactivating enzyme and dimers in U V-endonuclease-treated DNA is apparently normal, the light-dependent repair step either fails to occur or proceeds at a very low rate. Hence, besides the requirement of a minimum chain length for the function of the photoreactivating enzyme. there is the additional restriction on the position of the dimer in a polynucleotide strand.Finally, rough approximations of the rate constants. k , and k,. for the U V endonuclease indicate that the in uitro UV-endonuclease-dimer complex is relatively unstable, with dissociation of the complex being more probable than hydrolysis of the phosphodiester bond.
1968).Recently, the use of light flashes has allowed detailed characterization of the dark and light 371 372 M. H. PATRICK and HELGA HARM dependent reaction steps under varying experimental conditions, indicating many similarities between the action of yeast PRE in uitro and Escherichia coli PRE in uiuo and permits some general conclusions concerning the mechanisms of the photoenzymatic process (W. Harm et al., 1971).Because pyrimidine dimers are primarily responsible for the lethal effects of far UV, it is not surprising that there is a large overlap in the actions of photoenzymatic repair and ERR. The results presented in this paper show that, like the PRE, the M . luteus endonuclease recognizes and acts upon all types of cyclobutyl pyrimidine dimers in DNA but not other known UV-induced photoproducts. Furthermore, the two reactions can be mutually exclusive: single-strand nicks introduced by the endonuclease effectively preclude PR: conversely, formation of a PRE-dimer complex can prevent nicking by the UV endonuclease. This has permitted estimation of the constants associated with binding and dissociation of the endonuclease.
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