The ends of the affair: capping and polyadenylation.

Nearly all mRNAs are post-transcriptionally modified at their 5' and 3' ends, by capping and polyadenylation, respectively. These essential modifications are of course chemically quite distinct, as are the enzymatic complexes responsible for their synthesis. But recent studies have uncovered some similarities as well. For example, both involve entirely protein machinery, which is now the exception rather than the rule in RNA processing and modification reactions, and the two reactions share one important factor, namely RNA polymerase II. In this brief review, we describe progress in understanding the enzymes and factors that participate in these two processes, highlighting the evolutionary conservation, from yeast to humans, that has become apparent.