Genic Elements Research Articles

Duplicated genes producing transposable controlling elements for the mating-type differentiation in Saccharomyces cerevisiae.

Mutation of the two homothallic genes, HML alpha/HMLa and HMRa/HMR alpha, in homothallic strains of Saccharomyces cerevisiae was studied. Of 11 mutants of the HML alpha gene, eight were due to a phenotypic mutation from HML alpha to HMLa, i.e., a mutation causing a change in function of the original HML allele to that of the other HML allele (functional mutation), and three were due to a defective mutation at the HML alpha gene, i.e., a mutation causing a nonfunctional allele (nonfunctional mutation). All 14 mutants of the HMRa gene, on the other hand, were due to a phenotypic mutation from HMRa to HMR alpha i.e., a functional mutation. Phenotypic reverse mutations, i.e., HMLa to HML alpha and HMR alpha to HMRa, were also observed in the cultivation of EMS (ethyl methanesulfonate) treated spores having the HO HMR alpha HMLa genotype. Mutation from heterothallic cells to homothallism was observed in a nonfunctional mutant of the HML alpha gene, by mutagenesis with EMS, but not in the functional mutants of the HML alpha and HMRa genes or in the authentic strains having the alpha HO HMR alpha HML alpha (alpha Hp) and a HO HMRa HMLa (a Hq) genotypes. These observations suggest that the functional mutation is not caused by the direct mutation from a homothallic allele to the opposite, but by replacement of a transposable genic element produced from a homothallic locus with a region of a different homothallic locus. These observations also support the controlling-element model and the cassette model, which have been proposed to explain the mating-type differentiation by the homothallic genes.

Linked Genes, Pseudoalleles and Blood Groups

The theory of closely or absolutely linked genes and of "pseudoallelic" genic elements as applied to blood groups is critically examined both at the level of the antigen and at the level of the gene. The most complex system of blood groups recognized to date in any species, namely the "B" system of cattle, is used as a model throughout the discussion. Comparisons are drawn between certain features of the B system and the Rh system of man in regard to the kind of explanation involved in the "CDE" symbolism of the Rh blood groups. At the outset it is pointed out that the notion of linked genes and/or pseudoalleles traces to the erroneous impression that a single specificity is the definitive characteristic of an antigen. Hence, the theory of linked genes has constituted a problem in the study of blood groups only insofar as it has led to confusion in interpreting serological results. The hypothesis of multiple alleles proposed to explain the more complex systems of blood groups still exist and can be maintained even though the time may come when "crossover" mutants are recognized and differentiated from mutations in these alleles caused by other forces.