RNA polymerase-specific nucleosome disruption by transcription in vivo

The nucleosomal chromatin structure within genes is disrupted upon transcription by RNA polymerase II. To determine whether this disruption is caused by transcription per se as opposed to the RNA polymerase source, we engineered the yeast chromosomal HSP82 gene to be exclusively transcribed by bacteriophage T7 RNA polymerase in vivo. Interestingly, we found that a fraction of the T7-generated transcripts were 3' end processed and polyadenylated at or near the 3' ends of the hsp82 and the immediately downstream CIN2 genes. Surprisingly, the nucleosomal structure of the T7-transcribed hsp82 gene remained intact, in marked contrast to the disrupted structure generated by much weaker, basal level transcription of the wild type gene by RNA polymerase II under non-heat shock conditions. Therefore, disruption of chromatin structure by transcription is dependent on the RNA polymerase source. We propose that the observed RNA polymerase dependence for transcription-induced nucleosome disruption may be related either to the differential recruitment of chromatin remodeling complexes, the rates of histone octamer translocation and nucleosome reformation during polymerase traversal, and/or the degree of transient torsional stress generated by the elongating polymerase.