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First published online September 25, 2009
doi: 10.1242/10.1242/dev.027714

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Smicl is required for phosphorylation of RNA polymerase II and affects 3'-end processing of RNA at the midblastula transition in Xenopus

¹ Wellcome Trust/CR-UK Gurdon Institute and Department of Zoology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK.
² Wellcome Trust/CR-UK Gurdon Institute and Department of Genetics, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK.
³ Medical Research Council National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK.

^* Author for correspondence (jim.smith{at}nimr.mrc.ac.uk)

Accepted 13 August 2009

Smicl (Smad-interacting CPSF 30-like) is an unusual protein that interacts with transcription factors as well as with the cleavage and polyadenylation specificity factor (CPSF). Previous work has shown that Smicl is expressed maternally in the Xenopus embryo and is later required for transcription of Chordin. In this paper we search for additional targets of Smicl. We identify many genes whose onset of expression at the midblastula transition (MBT) requires Smicl and is correlated with the translocation of Smicl from cytoplasm to nucleus. At least one such gene, Xiro1, is regulated via 3'-end processing. In searching for a general mechanism by which Smicl might regulate gene expression at the MBT, we have discovered that it interacts with the tail of Rpb1, the largest subunit of RNA polymerase II. Our results show that Smicl is required for the phosphorylation of the Rpb1 tail at serine 2 of the repeated heptapeptide YSPTSPS. This site becomes hyperphosphorylated at the MBT, thus allowing the docking of proteins required for elongation of transcription and RNA processing. Our work links the onset of zygotic gene expression in the Xenopus embryo with the translocation of Smicl from cytoplasm to nucleus, the phosphorylation of Rpb1 and the 3'-end processing of newly transcribed mRNAs.

Key words: Xenopus, MBT, Smicl, Polyadenylation, RNA polymerase II phosphorylation, CCCH zinc finger

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