Several metazoan splicing factors are characterized by ribonucleoprotein (RNP) consensus sequences and arginine-serine repeats (RS domain) which are essential for their function in splicing. These include members of the SR-protein family (SC35, SF2/ASF), the U1 small nuclear (sn) RNP protein (U1-70K) and the U2 snRNP auxiliary factor (U2AF). SR proteins are phosphorylated in vivo and the phosphorylation state of U1-70K's RS domain influences its splicing activity. Here we report the purification of a protein kinase that is specific for SR proteins and show that it is DNA topoisomerase I. This enzyme lacks a canonical ATP-binding motif but binds ATP with a dissociation constant of 50 nM. Camptothecin and derivatives, known to be specific inhibitors of DNA topoisomerase I, strongly inhibit the kinase activity in the presence of DNA and affect the phosphorylation state of SR proteins. Thus, DNA topoisomerase I may well be one of the SR protein kinases operating in vivo.

H19 is a paternally imprinted gene whose expression produces a 2.4 kb RNA in most tissues during development and in mammalian myoblastic cell lines upon di erentiation. Deletion of the active maternal allele of H19 and its¯anking regions in the mouse leads to biallelic methylation and loss of imprinting of the neighbouring Igf2 gene. The function of H19 RNA remains unknown and, although polysome-associated, the absence of a conserved open reading frame suggests that it does not encode a protein product. We describe a novel post-transcriptional regulation of H19 gene expression which, in spite of this lack of coding capacity, is dependent on translational activity. We show that stabilization of the RNA is solely responsible for its accumulation during in vitro muscle cell di erentiation. This conclusion is based on the ®nding that inhibition of protein synthesis results in a dramatic destabilization of H19 RNA in proliferating mouse C2C12 myoblastic cells but not in di erentiated cells, and on run-on experiments which showed that the rate of transcription of H19 RNA remains constant during muscle cell di erentiation. This mechanism could also be involved in H19 gene expression during mouse development in addition to its transcriptional activation which we have shown to occur.