Effect of transcription of yeast chromatin on DNA topology in vivo.

Coding regions of transcribed and non-transcribed genes typically differ in chromatin structure. However, it is not known what kind of alterations in nucleosome or chromatin structure these differences reflect. To determine whether changes in nucleosome topology accompany transcription, we introduced into yeast a multicopy plasmid bearing the gene coding for the heat shock protein HSP26. The plasmid-borne gene is assembled into chromatin, and is induced by heat shock in the same manner as the endogenous HSP26 gene. A small change in linking number in the HSP26 plasmid accompanies heat shock. This change is consistent with that previously reported for thermal untwisting of DNA in yeast chromatin, and is equivalent in magnitude to that observed in control plasmids which lack heat-shock response elements. These data indicate that no stable alteration in nucleosome topology accompanies transcription of the heat shock gene. Moreover, the kinetics of the observed changes in linking number indicate that topoisomerase relaxes the thermally induced torsional stress in 1-5 min. We conclude that if alterations in nucleosome topology accompany polymerase passage, recovery must take place within this time period.