A MAMMALIAN ACTIVITY REQUIRED FOR THE 2ND STEP OF PREMESSENGER RNA SPLICING

Splicing of precursors to messenger RNAs occurs via a two-step mechani sm. In the first step, the 5'-exon is released concomitant with the pr oduction of a lariat intermediate, and in the second step, the exons a re joined, releasing the intron in the form of a lariat product. Sever al gene products of the yeast Saccharomyces cerevisiae have been shown to be required exclusively for the second step. Although mammalian pr oteins have been implicated in the second step of splicing, none have been shown to act only at this step. We identify here the first mammal ian activity shown to be exclusively required far the second step. The activity was shown to increase by 5-fold the rate for this splicing s tep, whereas it had no effect on the rate of the first step. The activ ity was not affected by treatment with micrococcal nuclease, whereas i t is sensitive to heating to 55 degrees C, suggesting that it is not d ependent on an RNA, but more likely is a protein. The second step acti vity was separated from other factors required for the first step and from PSF, a splicing factor thought to have a second step activity. Th e activity does not require ATP hydrolysis, suggesting that it acts at a late stage of the second step of splicing.