We have examined whether dissociation of the histone octamer is required for 
elongation of RNA transcripts through arrays of nucleosome cores in vitro. 
Control or dimethyl suberimidate-crosslinked histone octamers were reconstituted 
onto supercoiled, closed circular pT207-18 DNA, which contains tandemly repeated 
207-base-pair (bp) 5S rDNA nucleosome positioning sequences inserted between the 
T7 and SP6 transcription promoters of pGEM-3Z. Double label transcription 
experiments showed that there was little or no effect of extensive crosslinking 
of the histone octamers on transcription initiation and elongation by T7 RNA 
polymerase in vitro. Continuous regularly spaced linear arrays of either 
crosslinked or control nucleosome cores were obtained by digesting reconstituted 
nucleosomal pT207-18 templates with Dra I, a site that is protected from 
digestion by the presence of positioned nucleosome cores in the 207-pb sequence. 
After in vitro transcription with T7 RNA polymerase, an RNA ladder with 
207-nucleotide spacing was obtained from templates reconstituted both with 
crosslinked and with control histone octamers, demonstrating clearly that 
neither partial nor complete dissociation of the histone octamer is essential 
for transcription elongation through arrays of nucleosome cores in vitro.