ENZYMES INVOLVED IN DNA REPAIR AND MEIOSIS

从DNA修复和重组的研究来看，很明显核酸酶

From studies of DNA repair and recombination, it is apparent that nucleases play a major role in both of these processes. In the yeast S. cerevisiae the RAD52 gene is essential for the repair of DNA double-strand breaks (DSB), mitotic recombination and for the successful completion of meiosis. Although the function of RAD52 remains unclear, RAD52 may play a role in controlling the levels of a 72 kD nuclease (RhoNUC). From Western blot analysis, the levels of RhoNUC are greatly decreased in rad52 mutants suggesting that this nuclease is under the control of RAD52. The gene encoding RhoNUC (RNC1) has been cloned and sequenced. OFAGE analysis placed the gene encoding RhoNUC on chromosome XI. Examination of the deduced primary sequence reveals that RhoNUC is a chimeric protein with the N-terminal region exhibiting extensive homology with the ras/rho superfamily of GTP binding proteins while the C-terminus appears to encode the nuclease function. Hence, this protein has been named RhoNUC for rho- associated nuclease. Analysis of RNC1::lacZ gene fusions reveals that the control of RhoNUC by RAD52 does not occur at the level of transcription suggesting that the control is at the post-translational level. Although overexpression of RNC1 yields no significant genetic phenotype, the overexpression does cause the cells to enlarge and their nuclei appear enlarged and diffuse. Interestingly, strains carrying deletions for both RAD52 and RNC1 have nearly wild-type levels of direct repeat recombination when measured either on plasmids or on the chromosome (strains carrying only a deletion of RAD52 exhibit recombination levels approximately 100- fold lower than wild-type). From these observations, we believe that RhoNUC represents a newly identified class of proteins exhibiting both a cellular signalling function (G-protein activity) and a DNA metabolic activity (nuclease activity involved in recombination).