MBR1 and MBR3, two related yeast genes that can suppress the growth defect of hap2, hap3 and hap4 mutants.

Two new yeast genes, named MBR1 and MBR3, were isolated as multicopy suppressors of the growth defect of a strain lacking the HAP2 transcriptional activator. Both genes when overexpressed can also suppress the growth defect of hap3 and hap4 null mutants. However, overexpression of MBR1 cannot substitute for the HAP2/3/4 complex in activation of the CYC1 gene. Nucleotide sequencing of MBR1 and MBR3 revealed that these two genes encode serine-rich, hydrophilic proteins with regions of significant homology. The functional importance of one of these conserved regions was shown by mutagenesis. Disruption of MBR1 leads to a partial growth defect on glycerol medium. Disruption of MBR3 has no major effect but the double disruptant shows a synthetic phenotype suggesting that the MBR1 and MBR3 gene products participate in common function.