TUP1 disruption reveals biological differences between MATa and MATα strains of Cryptococcus neoformans

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Abstract
Cryptococcus neoformans exists in two mating types MATa and MATalpha. Although the morphology, growth characteristics and genetic segregation patterns among MATa and MATalpha strains are indistinguishable in the laboratory, the predominance of MATalpha strains in nature suggests that MATalpha strains are better suited for survival in nature. We disrupted the TUP1 gene, a global repressor, to find the possible biological differences in congenic MATalpha and MATa cells of C. neoformans. Disruption of TUP1 affected neither the yeast nor the hyphal cell morphology but resulted in a similar reduction of mating frequencies in both MATalpha and MATa cells. Disruption of TUP1, however, functionally manifested itself in several mating type-dependent phenotypes: (i) MATalpha cells became more sensitive to 0.8 M KCl while MATa cells showed no change in sensitivity, (ii) a temperature-dependent growth reduction was exhibited at both 30 degrees C and 25 degrees C in MATa but a similar growth reduction was not observed in MATalpha cells until the temperature was lowered to 25 degrees C and (iii) the transcriptional level of genes in several different biological pathways was markedly altered in a mating type-dependent manner. This work is the first case in which non-mating-related biological differences are observed between two congenic mating partners in yeast.