Transcription initiation by RNA polymerase II is inhibited by S-adenosylhomocysteine.

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Abstract
Most eukaryotic mRNA are blocked at their 5'' termini by guanylylation and methylation. These cap structures were shown to play important roles in increasing the stability and translatability of mRNA. These modifications may occur extremely early in the synthesis of RNA transcripts by RNA polymerase II. S-adenosylhomocysteine (AdoHcy), both a product and an inhibitor of transmethylation reactions, inhibits transcription intitiation by RNA polymerase II, but not by RNA polymerase III, in a HeLa whole-cell lysate. AdoHcy must be present during initiation to inhibit transcription and does not affect elongation by RNA polymerase II or the stability of the resultant transcript. AdoHcy does not inhibit transcription by purified HeLa RNA polymerase II. Formation of the 5''-cap structure is evidently coupled to initiation of transcription and is consistent with a close association between the capping enzymes and RNA polymerase II at the time of initiation.