Correlation of endonuclease activity at meiotic prophase and cofactor-induced genetic recombination in the basidiomycete Coprinus cinereus

Abstract

An endonuclease was purified from the basidiocarps of the fungus Coprinus cinereus during late premeiotic S-phase, karyogamy, and pachytene stages of meiosis. This endonuclease causes single-strand nicks on supercoiled PM2 DNA. Its activity is dependent on cofactors such as Mg2+ and Ca2+, and is enhanced by hexaminecobalt chloride and dimethylsulfoxide. Treatments with increasing cofactor concentration elicit increasing enzyme activity in vitro and increasing genetic recombination in vivo. The percent recombination that can be induced by cofactor treatment during late premeiotic S-phase, karyogamy and pachytene is correlated with the amount of endonuclease that can be extracted from the respective stages. These results suggest that Mg2+ and Ca2+ induce genetic recombination by increasing the endonuclease activity that creates nicked and gapped DNA substrates for ensuing recombination events. We believe that the meiotic endonuclease of Coprinus is involved in the early phase of genetic recombination. Cold temperature treatment is known to retard repair activity at pachytene and causes a threefold increase in recombination. Double treatments involving Mg2+ and Ca2+ at karyogamy followed by cold temperature at pachytene result in an additive (more than fourfold) increase in recombination frequency. The peak concentration of the nicking endonuclease is found at late premeiotic S and early karyogamy, whereas the peak activity of the repairing DNA polymerase b is found at late pachytene. The staging of these two enzymes is in good accord with the genetic data.Key words: Coprinus, endonuclease, meiosis, genetic recombination, Z-DNA.