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Abstract

All eukaryotic protein-coding genes are transcribed by RNA polymerase II (pol II), and each mRNA is the product not only of transcription but a variety of pre-mRNA processing events. In humans, every pre-mRNA acquires a methyl-guanosine cap at its 5` end, and nearly every transcript is internally spliced and polyadenylated at its 3` end. The purpose of this chapter is to place these three major pre-mRNA processing steps within the context of the three dimensional space of the cell nucleus. Because capping, splicing and polyadenylation at least begin during RNA synthesis, these reactions occur largely at sites of gene transcription, which are distributed throughout the nucleus and not localized to particular domains or substructures. Splicing and polyadenylation often continue post-transcriptionally, most likely in the interchromatin space. In addition, pre-mRNA processing factors are components of a number of subnuclear structures, such as Cajal Bodies and Cleavage Bodies, suggesting that some functions related to pre-mRNA processing are compartmentalized within the nuclear landscape.