The C-terminal domain (CTD) of RNA polymerase II is phosphorylated during the transcription cycle by three cyclindependent kinases (CDKs): CDK7, CDK8, and CDK9. CDK9 and its interacting cyclin T partners belong to the positive transcription elongation factor b (P-TEFb) complexes, which phosphorylate the CTD to promote transcription elongation. We report that Arabidopsis thaliana CDK9-like proteins, CDKC;1 and CDKC;2, and their interacting cyclin T partners, CYCT1;4 and CYCT1;5, play important roles in infection with Cauliflower mosaic virus (CaMV). cdkc;2 and cyct1;5 knockout mutants are highly resistant and cdkc;2 cyct1;5 double mutants are extremely resistant to CaMV. The mutants respond normally to other types of plant viruses that do not replicate by reverse transcription. Expression of a reporter gene driven by the CaMV 35S promoter is markedly reduced in the cdkc;2 and cyct1;5 mutants, indicating that the kinase complexes are important for transcription from the viral promoter. Loss of function of CDKC;1/CDKC;2 or CYCT1;4/CYCT1;5 results in complete resistance to CaMV as well as altered leaf and flower growth, trichome development, and delayed flowering. These results establish Arabidopsis CDKC kinase complexes as important host targets of CaMV for transcriptional activation of viral genes and critical regulators of plant growth and development.
INTRODUCTIONThe transcription of protein-coding genes in eukaryotes is performed by RNA polymerase II (RNAP II) and forms a so-called transcription cycle that includes preinitiation, initiation, promoter clearance, elongation, and termination (Sims et al., 2004). The transcription cycle starts with the assembly of the preinitiation complex at the promoter, which includes the general transcription factors TFIID, TFIIB, TFIIE, and TFIIH as well as RNAP II. Assembly of the preinitiation complex is followed by ATPdependent melting of the double-stranded DNA (dsDNA) template and formation of an open complex between RNAP II and the DNA template. Once the open complex is established, transcription initiation occurs and RNAP II is allowed to clear the promoter and engaged to make the transition from initiation to elongation. Transcription elongation is a dynamic and highly regulated process that coordinates downstream events such as capping and splicing of primary transcripts. The final step in the cycle is transcription termination. At this stage, the mRNA is cleaved, polyadenylated, and transported to the cytoplasm for translation.The transcription cycle involving RNAP II is accompanied by another cycling event: phosphorylation of the C-terminal domain (CTD) of the largest subunit of RNAP II (Sims et al., 2004). The RNAP II CTD contains multiple repeats of the heptapeptide sequence YSPTSPS (26 such repeats in yeast, 32 in Caenorhabditis elegans, 45 in Drosophila, and 52 in mammals). RNAP II in the preinitiation complex is unphosphorylated or hypophosphorylated, whereas transcription-competent RNAP II is heavily phosphorylated on its CTD, with Ser-2 and Ser-5 as the major...
