REGULATED EXPRESSION AND RNA PROCESSING OF TRANSCRIPTS FROM THE SRP20SPLICING FACTOR GENE DURING THE CELL-CYCLE

Eukaryotic splicing factors belonging to the SR family are essential s plicing factors consisting of an N-terminal RNA-binding region and a C -terminal RS domain. They are believed to be involved in alternative s plicing of numerous transcripts because their expression levels can in fluence splice site selection. We have characterized the structure and transcriptional regulation of the gene for the smallest member of the SR family, SRp20 (previously called X16). The mouse gene encoding SRp 20, termed Srp20, consists of one alternative exon and six constitutiv e exons and was mapped to a 2-centimorgan interval on chromosome 17. W hen cells are transfected with SRp20 genomic DNA, both standard and al ternatively spliced transcripts and corresponding proteins are produce d. Interestingly, in starved (G(0)) cells, the amount of SRp20 mRNA co ntaining the alternative exon is large, whereas the amount of the stan dard SRp20 mRNA without the alternative exon is small. When starved ce lls are stimulated with serum, the alternative form is lost and the st andard form is induced. These results suggest that splicing could be r egulated during the cell cycle and that this could be, at least in par t, due to regulated expression of SR proteins. Consistent with this, e xperiments with synchronized cells showed an induction of SRp20 transc ripts in late G(1) or early S. We have also characterized the promoter of SRp20. It lies within a CC-rich CPG island and contains two consen sus binding sites for E2F, a transcription factor thought to be involv ed in regulating the cell cycle. These motifs may be functional since reporter constructs with the SRp20 promoter can he stimulated by cotra nsfection with E2F expression plasmids.