Reversible
inactivation of TRP5, CYH2 and MET13 dominate alleles in
heterozygous yeast cells.
Maxim Ivanov,
Anna Aksenova, Ludmila Mironova
Department of Genetics, St. Petersburg State University, Universitetskaya nab,
Saint-Petersburg, 199034, Russia (aranarh@btc.bio.pu.ru)
An unusual behavior of markers of chromosome VII left arm is described. The diploid used was heterozygous for auxotrofic mutations trp5 and met13-A1 and for cycloheximide resistance mutation cyh2-1. A majority of clones selected in the mitotic progeny of this diploid as cycloheximide resistant manifested the recessive phenotype for two other markers as well. Further analysis of these clones demonstrated that they were not homozygous or hemizygous for recessive alleles of all three genes, as could be expected, but retained wild type alleles in transiently inactive state. This conclusion follows from: (i) the ability of clones to revert to Met+ phenotype on the C-met medium (met13-A1 is a stable allele, which differs from the wild type allele by a few mutations); (ii) the dominant phenotype of Met+ clones for CYH2 or TRP5, or both, confirming non-mutational nature of Met+ phenotype and indicating that reactivation of CYH2 and/or TRP5 follows MET13 reactivation. (iii) the data of tetrad analysis of diploid clones manifesting the recessive phenotype (i.e. auxotrofic for methionine and tryptophan and cycloheximide resistant). The 2:2 ratio for all three markers has been observed in the meiotic progeny of these diploids. Thus, the epigenetic inactivation of the stretch of chromosome VII is supported only in mitotic divisions, but not in meiosis. The molecular mechanisms of the phenomenon remain to be established. The work is supported by RFBR 02-04-49699 and CRDF ST-012 grants.