Zvi Pasman | PubMed Health

PubMed2002

Reaction pathways in transcript elongation.

Transcription of DNA into RNA is a central part of gene expression, and is highly regulated in all organisms. In order to approach transcription control systems on a molecular basis we must understand the mechanisms used by the transcription complex to discharge its various functions, which include transcript initiation, elongation, editing, and termination. In this article we describe recent progress in sorting out the multiple reaction pathways that are, at least in principle, available to the transcription complex at each DNA template position, and show how transcription control systems partition active complexes into these pathways. Understanding these regulatory processes requires an elucidation of the molecular details of how sequence- and factor-dependent changes in the conformations, stabilities, and reaction rates of the complexes determine function. Recent progress in unraveling these issues is summarized in this article and emerging principles that govern the regulation of the elongation phase of transcription are discussed.

DNA-Directed RNA Polymerases↗

PubMed2002

Active Escherichia coli transcription elongation complexes are functionally homogeneous.

The elongation phase of RNA transcription represents a major target for the regulation of gene expression. Two general classes of models have been proposed to define the dynamic properties of transcription complexes in the elongation phase. Stable heterogeneity models posit that the ensemble of active elongation-competent complexes consists of multiple distinct and stable forms that are specified early in the transcription cycle and isomerize to other forms slowly. In contrast, equilibrium or rapid interconversion models require that active elongation complexes interconvert rapidly on the time-scale of single nucleotide addition. Measurements of transcription termination efficiency (TE) can be used to distinguish between these models, because stable heterogeneity models predict that the termination-resistant fraction of an elongation complex population should be enriched after transcription through an upstream terminator, leading to a decreased TE at downstream terminators. In contrast, rapid interconversion models require that the population of active (elongation-competent) complexes equilibrate after transcription through each terminator and, therefore, that the value of TE observed at identical upstream and downstream terminators should be the same. We have constructed transcription templates containing multiple identical terminators and found no significant changes in TE with terminator position along the template. Various other forms of upstream treatment of elongation complex populations also were used to attempt to fractionate the complexes into functionally different forms. None of these treatments changed the apparent TE at downstream terminators. These results are consistent with a rapid interconversion model of transcript elongation. The consequences of these results for the regulation of gene expression are discussed.

Bacteriophage lambda↗