MUTATIONS CAUSING HIGH BASAL LEVEL TRANSCRIPTION THAT IS INDEPENDENT OF TRANSCRIPTIONAL ACTIVATORS BUT DEPENDENT ON CHROMOSOMAL POSITION INSACCHAROMYCES-CEREVISIAE

Two single (bel2 and bel4) and two double (bel3 bel7 and bel5 bel6) mu tations causing enhanced transcription of a gene fusion, consisting of the open reading frame of PHO5 connected to the HIS5 promoter (HIS5p) integrated at the ura3 or leu2 locus, were isolated from a gcn4-disru pted mutant of Saccharomyces cerevisiae. The PHO5 gene, encoding repre ssible acid phosphatase, in the HIS5p-PHO5 construct was derepressed u nder amino acid starved conditions by the action of the transcriptiona l activator Gcn4p. The bel mutants showed temperature-sensitive cell g rowth and/or cell aggregation. All the mutants except bel4 also showed high levels of transcription of an intact PHO5 DNA integrated at the URA3 locus in the absence of the cognate transcriptional activator, Ph o4p, and in the absence of upstream activating sequences of PHO5. The HIS5 and PH05 genes at their original chromosomal positions were, howe ver, not affected by the bel2 mutation. The BEL2 gene was found to be identical with SIN4/TSF3, mutations in which cause high levels of tran scription of the HO and GAL genes in the absence of their respective t ranscriptional activators, Swi5p and Gal4p. The effect of the bel2/sin 4/tsf3 mutation on PH05 transcription was additive with the Pho4p func tion. Thus the effect of the bel2/sin4/tsf3 mutation is dependent on t he position of PHO5 in the chromosome and independent of Pho4p and Gen 4p activation.