SOMA-SPECIFIC EXPRESSION AND CLONING OF PSI, A NEGATIVE REGULATOR OF P-ELEMENT PRE-MESSENGER-RNA SPLICING

PSI is an RNA-binding protein involved in repressing splicing of the P element third intron in Drosophila somatic cell extracts. PSI produce d in bacteria restores splicing inhibition to an extract relieved of i nhibitory activity, indicating that PSI plays a direct role in somatic inhibition. Sequence analysis of cDNAs encoding PSI reveals three KH RNA-binding domains, a conserved motif also found in the yeast splicin g regulator MER1. Notably, PSI is expressed highly in somatic embryoni c nuclei but is undetectable in germ-line cells. In contrast, hrp48, a nother protein implicated in somatic inhibition, is found in the nucle us and cytoplasm of both tissues. The splicing inhibitory properties a nd soma-specific expression of PSI may be sufficient to explain the ge rm-line-specific transposition of P elements.