ABSTRACT
CPF1 is an abundant basic-helix-loop-helix-ZIP protein that binds to the CDEI motif in Saccharomyces cerevisiae centromeres and in the promoters of numerous genes, including those encoding enzymes of the methionine biosynthetic pathway. Strains lacking CPF1 are methionine auxotrophs, and it has been proposed that CPF1 might positively influence transcription at the MET25 and MET16 genes by modulating promoter chromatin structure. We test this hypothesis and show that the regions surrounding the CDEI motifs in the MET25 and MET16 promoters are maintained in a nucleosome-free state and that this requires the entire CPF1 protein. However, the chromatin structure around the CDEI motifs does not change on derepression of transcription and does not correlate with the methionine phenotype of the cell. An intact CDEI motif but not CPF1 is required for transcriptional activation from a region of the MET25 upstream activation sequence. Our results suggest that CPF1 functions to modulate chromatin structure around the CDEI motif but that these changes at the MET25 and MET16 promoters do not explain how CPF1 functions to maintain methionine-independent growth. The presence of CPF1-dependent chromatin structures at these promoters leads to a weak repression of transcription.
