Specificity of chromatin transcription in vitro. Anomalies due to RNA-dependent RNA synthesis

In the presence of Mn2+, globin mRNA can be transcribed into a partial RNA copy by Escherichia coli RNA polymerase. This process also occurs when the mRNA is transcribed together with chromatin. A fraction, at least, of the newly synthesized RNA copy (anti-globin RNA) can serve as a template for the synthesis of globin sequences of the same polarity as the original mRNA. This process is sufficient to explain the specific synthesis of a subset of the globin RNA on mouse foetal liver chromatin. It also accounts for the synthesis of double-stranded RNA sequence by E. coli RNA polymerase, on chromatin as well as on pure mRNA. Results are presented suggesting that the poly(A) tract of the mRNA could be preferentially transcribed. In the presence of Mg2+, the RNA-dependent transcription is strongly inhibited, as well as the synthesis of double-stranded RNA. Under these conditions, the transcription on chromatin appears to be largely DNA dependent, and the synthesis of globin sequences is completely asymmetric. Spermine (0.3 mM) seems to improve the specificity of transcription. The transcription of chromatin in vitro is thus largely dependent on the nature of the divalent cation present in the in the incubation mixture.