It has been reported that two upstream activation sites, UAS1 and UAS2, exist in the 5' non-coding region of the STA1 gene of Saccharomyces cerevisiae var. diastaticus. Based on studies using a UAS1STA1-CYC1-lacZ fusion, we divided UAS1 into two subsites, UAS1-1 and UAS1-2. The activation of the CYC1 promoter by UAS1STA1 was repressed by glucose in the culture medium and by the STA10 gene. The MATa/MAT alpha mating type configuration did not, however, affect UAS1STA1 activation. The UAS1STA1-CYC1-lacZ expression system was used to study STA10 repression further. A mutant insensitive to STA10-dependent repression was isolated. This sns1 mutation was not linked to STA10 and partially overcame the repressive effect of STA10 at the transcriptional level. From a genomic library constructed in the UAS1STA1-CYC1-lacZ expression vector, the MSS1 locus (multicopy suppressor of sns1) was isolated. This suppression of the sns1 mutation by multiple copies of the MSS1 locus occurred at the transcriptional level. When a gene disruption experiment was performed to examine the effect of a mss1 mutation, the sns1 mss1 double mutants produced 4 times higher levels of STA1 transcripts in the presence of STA10 than did the sns1 strain. Data presented in this paper suggest that both SNS1 and MSS1 loci are involved in STA10-dependent repression.