The ribonucleic acid synthesized in vitro using a chromatin template is indistinguishable by nucleic acid hybridization analysis from ribonucleic acid produced in vivo in the homologous tissue. Purified deoxyribonucleic acid supports a higher rate of ribonucleic acid synthesis than chromatin preparations. This is due to the fact that there is a restricted number of sites transcribed from chromatin. Within the limits of sensitivity of the hybridization assay this restriction is tissue specific. This specificity appears to be independent of the source of the ribonucleic acid polymerase as comparable results are obtained with Escherichia coli and mouse enzyme. Indirect evidence indicates that the synthesis of new molecules is initiated during in vitro transcription. The fact that deoxyribonucleic acid-ribonucleic acid hybridization is unable to distinguish between ribonucleic acid molecules having a high degree of similarity in base composition and that incubation times used during the hybridization were too short to permit hybridization of ribonucleic acid molecules originating from sites of unique nucleotide sequence requires that strict limitations be placed on the interpretation of these results. In spite of these qualifications, it is evident that a high degree of tissue specificity is retained by isolated chromatin with respect to the synthesis of ribonucleic acid in vitro. It would appear that the elements determining the differential transcription of ribonucleic acid reside within the macromolecular configuration of the interphase chromosomes.
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