We have used a linear DNA template (239 bp) containing a nucleosome positioning 
sequence (NX1) downstream of the T7 RNA polymerase promoter to study the 
mechanism of transcription elongation through a nucleosome. Under ionic strength 
approaching physiological conditions we have observed that transcription causes 
nucleosome dissociation and histone redistribution within the template. We have 
examined the role of the different elements that, in principle, could induce 
nucleosome dissociation during transcription. The high affinity of histones for 
single-stranded DNA observed in titration experiments performed using the 
purified (+) and (-) strands of the NX1 fragment suggests that nucleosome 
dissociation is not due to the formation of segments of single-stranded DNA by 
RNA polymerase in the elongation process. Furthermore, our results show that 
although RNA can interact with core histones, the synthesized RNA is not bound 
to the histones dissociated by transcription. Our results indicate that core 
histones released during transcription can be bound to naked DNA and chromatin 
(with or without histones H1-H5). From the dynamic properties of excess histones 
bound to chromatin, we suggest a nucleosome transcription mechanism in which 
displaced histones are transiently bound to chromatin and finally are 
reassembled with DNA after the passage of the polymerase.