ALL 4 CORE HISTONE N-TERMINI CONTAIN SEQUENCES REQUIRED FOR THE REPRESSION OF BASAL TRANSCRIPTION IN YEAST

Nucleosomes prevent the recognition of TATA promoter elements by the b asal transcriptional machinery in the absence of induction. However, w hile Saccharomyces cerevisiae histones H3 and H4 contain N-terminal re gions involved in the activation and repression of GAL1 and in the rep ression of heterochromatin-like regions, the sequences involved in rep ressing basal transcription have not yet been identified. Here, we des cribe the mapping of new N-terminal domains, in all four core histones (H2A, H2B, H3 and H4), required for the repression of basal, uninduce d transcription. Basal transcription was monitored by the use of a GAL 1 promoter-URA3 reporter construct whose uninduced activity can be det ected through cellular sensitivity to the drug, 5-fluoroorotic acid. W e have found for each histone that the N-terminal sequences repressing basal activity are in a short region adjacent to the structured a-hel ical core. Analysis of minichromosome DNA topology demonstrates that t he basal domains are required for the proper folding of DNA around the nucleosomal particle. Deletion of the basal domain at each histone si gnificantly decreases plasmid superhelical density, which probably ref lects a release of DNA from the constraints of the nucleosome into the linker region. This provides a means by which basal factors may recog nize otherwise repressed regulatory elements.