UME6 IS A KEY REGULATOR OF NITROGEN REPRESSION AND MEIOTIC DEVELOPMENT

This report describes the identification, cloning, and molecular analysis of UME6 (CAR80/CARGRI), a key transcriptional regulator of early meiotic gene expression. Loss of UME6 function results in the accumulation of fully derepressed levels (70- to 100-fold increase above basal level) of early meiotic transcripts during vegetative growth. In contrast, mutations in five previously identified UME loci (UME1 to UME5), result in low to moderate derepression (2- to 10-fold increase) of early meiotic genes. The behavior of insertion and deletion alleles indicates that UME6 is dispensable for mitotic division but is required for meiosis and spore germination. Despite the high level of meiotic gene expression during vegetative growth, the generation times of ume6 mutant haploid and diploid cells are only slightly reduced. However, both ascus formation and spore viability are affected more severely. The UME6 gene encodes a 91-kD protein that contains a C6 zinc cluster motif similar to the DNA-binding domain of GAL4. The integrity of this domain is required for UME6 function. It has been reported recently that a mutation in CAR80 fails to complement an insertion allele of UME6. CAR80 is a gene required for nitrogen repression of the arginine catabolic enzymes. Here, through sequence analysis, we demonstrate that UME6 and CAR80 are identical. Analyses of UME6 mRNA during both nitrogen starvation and meiotic development indicate that its transcription is constitutive, suggesting that regulation of UME6 activity occurs at a post-transcriptional level.