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SEVERAL metazoan splicing factors are characterized by ribo-nucleoprotein (RNP) consensus sequences and arginine–serine repeats (RS domain)^1–8 which are essential for their function in splicing^5,9,10. 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 vivo¹¹ and the phosphorylation state of U1-70K's RS domain influences its splicing activity¹². 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 …