RNA Polymerases and Transcriptive Specificity in Eukaryotic Organisms

Abstract

Selective gene transcription implies an intriguing modality in specificity of the transcriptive system which is uncommon in most enzymatically catalyzed reactions (Fig. 1). For specific gene selection, recognition of initiator DNA sequences by the transcribing unit is required. On the other hand, transcription itself involves an indiscriminate copying of the DNA template. Finally a mechanism for termination probably involves some sensing of DNA sequence. These initiating, transcribing, and terminating modes of the transcriptive system require specificity-determining elements either in the chromosomal template, the RNA polymerase, or in another component of the transcribing system. Three models to explain the specificity are immediately apparent. In the template restriction model, the non-transcribed portions of the DNA are essentially sequestered from an indiscriminate RNA polymerase. Alternatively, one can envision multiple RNA polymerases with inherent specificity for initiating and terminating sequences. In this instance, the polymerases could exist in initiating transcribing, and terminating modes. This would probably require at least two conformational states (the initiating and terminating modes could be equivalent). Finally, the specificity of initiation and termination could be provided by specific external factors which would aid an indiscriminate polymerase in selecting sites for transcription.