THE RNA-POLYMERASE-II GENERAL ELONGATION COMPLEX

Eukaryotic messenger RNA (mRNA) synthesis is a complex multi-stage pro cess that requires the concerted action of many cellular factors to ge nerate a mature functional message. This elaborate process by RNA poly merase II (pol II) proceeds via multiple stages-preinitiation, initiat ion (Figure 1), promoter clearance, elongation (Figure 1) and terminat ion - which have come to be referred to collectively as the transcript ion cycle, Although the preinitiation and initiation stages of transcr iption have received the most attention during the past decade, the pa st few years have been a watershed for biochemical studies of the pol II elongation complex, Recent studies have demonstrated the existence of several families of pol II elongation factors and nuclear proteins that can govern the activity of pol II during mRNA chain elongation. N ew findings have revealed that the elongation stage of transcription i s a critical site for the regulation of gene expression. Evidence obta ined to date suggests that eukaryotes regulate elongation by both 'gen eral' and 'activator dependent' mechanisms, These mechanisms necessita te alteration of pol II's catalytic site, modification of chromatin st ructure, phosphorylation of the pol II carboxyl-terminal domain (CTD) and involvement of other components of the transcription machinery to increase the rate and efficiency of transcription elongation. This min ireview is an annotation on the recent progress in studies of the bioc hemical mechanism and molecular regulation of the elongation stages of eukaryotic mRNA synthesis. The recent developments that have guided o ur understanding and propelled current research on transcription elong ation by mammalian pol II will be described here.