The kinetics of synthesis and the metabolic stability of uncapped vesicular stomatitis virus (VSV) mRNA transcripts have been studied using techniques which clearly differentiate them from other RNA species. The triphosphate-initiating mRNA transcripts accumulate for at least 5 h during a typical transcription reaction. The great majority of these transcripts are smaller than a functional message and have been released from the template-transcriptase complex. Label that accumulates in them is stable and is not detectably diminished after a 1 h chase with unlabelled precursor. These kinetic properties are not those expected for active intermediates in mRNA synthesis and suggest that the uncapped transcripts are products of aborted transcription that accumulate during the transcriptive process. However, we cannot rule out that a small subset of these transcripts may be elongated into mature mRNA. Initiation of transcription at internal positions along the VSV genome is both frequent (one-half to one-sixth as frequent as initiations at the leader RNA gene) and site-specific (occurring only at the beginning of cistrons). The relevance of these results to the models for VSV transcription is discussed.


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