Reciprocal interferences between nucleosomal organization and transcriptional activity of the yeast SNR6 gene.

Recent work has demonstrated a repressive effect of chromatin on the transcription of the yeast SNR6 gene in vitro. Here, we show the relations between chromatin structure and transcriptional activity of this gene in vivo. Analysis of the SNR6 locus by micrococcal nuclease digestion showed a protection of the TATA box, nuclease-sensitive sites around the A and B blocks, and arrays of positioned nucleosomes in the flanking regions. Analysis of a transcriptionally silent SNR6 mutant containing a 2-bp deletion in the B block showed a loss of TATA-protection and rearrangement or destabilization of nucleosomes in the flanking regions. Hence, SNR6 organizes the chromatin structure in the whole region in a manner dependent on its transcriptional state. Transcriptional analysis was performed by use of maxi-gene SNR6 constructs introduced into histone-mutated strains. Chromatin disruption induced by histone H4 depletion stimulated the transcription of promoter-deficient, but not of wild-type SNR6 genes, revealing a competition between the formation of nucleosomes and the assembly of Pol III transcription complexes that was much in favor of transcription factors. On the other hand, amino-terminal mutations in histone H3 or H4 had no effect (H4) or only a moderate stimulatory effect (H3) on the transcription of promoter-deficient SNR6 genes.