The specificity of in vitro chromatin transcription

The in vitro transcription of chicken reticulocyte chromatin with E. coli RNA polymerase has been studied in several different ways. The amount of globin RNA sequences has been measured by hybridizing the transcript with globin cDNA; we show that under the proper conditions mercurated transcript RNA can be separated from endogenous RNA on sulfhydryl affinity columns. The amount of globin RNA in the transcript is approximately 20 fold greater than that from erythrocyte chromatin or reticulocyte DNA. Although these data could be used to support the hypothesis of specific transcription, we show by RNA/RNA self hybridization of the transcript (which is at least 50% symmetric) and by hybridization of the transcript to unique DNA in vast RNA excess that the bulk of the chromatin transcript differs little from the transcript of naked DNA. Several explanations for these apparently contradictory results are offered with the most likely one being compatible with random transcription of at least most of the sequences in the chromatin.