Transcription of multiple copies of the yeast GAL7 gene is limited by specific factors in addition to GAL4

High levels of the GAL7 gene in the yeast cell appear to titrate regulatory factors and to impair transcription of related sequences. To investigate the role that the GAL regulatory factors GAL4 and GAL80 have in this process we have compared the accumulation of mRNA transcribed from single-copy (plasmid-borne GAL7 and chromosomal GAL10) and high-copy (plasmid-borne GAL7) genes in several GAL regulatory mutants. Our results show that functional GAL4 gene product is required for induction of transcription from the single- and high-copy genes. In a strain containing the GAL4 gene fused to the high expression ADH1 promoter, glucose can replace galactose to induce high levels of transcription of GAL7 and GAL10 genes, although the kinetics of accumulation induced by the two sugars are distinctly different. In the presence of high levels of GAL4, maximum accumulation of mRNA from single and high copy genes is elevated two-fold; disruption of the gal80 gene in combination with high levels of GAL4 results in a further two-fold increase in transcription. In this genetic background, galactose-induced transcription of the high copy GAL7 gene results in a greater than 50-fold increase in the levels of GAL7 mRNA, representing 30%-50% of the total cellular mRNA. Our results are consistent with a cooperative effect of saturation of multiple GAL4 DNA binding sites and with a limiting factor, in addition to GAL4, that is required for transcription of the GAL genes.