The link between mRNA processing and transcription: communication works both ways

Many pre-mRNA processing events including 5' end capping, splicing out introns, and 3' end maturation by cleavage or polyadenylation occur while the nascent RNA chain is being synthesized by RNA polymerase II. As a consequence of this arrangement, the physiological substrate for most processing factors is not a solitary pre-RNA but instead a ternary complex comprising a growing RNA chain spewing from the exit channel of an RNA polymerase II molecule as it speeds along a chromatin template at 1000-2000 bases/min. mRNA processing factors make protein-protein contacts with elongating pol II in a complex we have dubbed the "mRNA factory," which carries out synthesis, processing, and packaging of the transcript. Recent studies have shown that the "mRNA factory" is a dynamic complex whose composition changes as it traverses the length of a gene. This complex is also the setting for a growing number of regulatory interactions, which influence the function of both the processing and transcription machineries.