The largest subunit of eukaryotic RNA polymerase II, Rbp1 possesses an unusual C-terminal domain (CTD) consisting of multiple tandem repeats of a heptapeptide with consensus sequence Tyr1-Ser2-Pro3-The4-Ser5-Pro6-Ser7. The CTD forms an important link between transcription and RNA processing. The basis of this function is physical interactions between the CTD and various CTD-binding domains of transcription factors involved in RNA processing. Individual residues within the CTD repeat can undergo posttraslational modifications which result in a variety of distinct CTD patters to be recognized by different factors. This allows precise regulation of the recruitment of various transcription factors and hence cooperation between transcription and RNA processing. CTD-interacting domains (CIDs) form a conserved family of CTD-binding domains found in several transcription factors. Despite high uniformity of CIDs at all structural levels, available studies of yeast CIDs:CTD complexes show that CIDs differ in regard of their specificity towards certain CTD modification patterns. For instance the CID of Nrd1p (involved in transcription termination of non coding RNAs and RNA processing) recognizes CTD phosphorylated on Ser5 within two consecutive repeats. Whereas CIDs of Pcf11p and Rtt103p (involved in transcription termination and processing of mRNA) specifically interact with the CTD phosphorylated on Ser2 within two consecutive repeats; although each uses different mechanism for the binding. We have recently found that the CID domain of human protein RPRD2 shows specificity for rather unusual CTD phosphorylation pattern comparing to other CIDs. Moreover this interaction is characterized by so far the highest affinity towards CTD among CIDs. The aim of this work is to explain structural basis of this unusual CID:CTD interaction using Fluorescence anisotropy measurements, nuclear magnetic resonance and complementary molecular biology techniques. These data can further improve our understanding of the CTD function and provide useful information for the functional characterization of RPRD2. Literatura: Becker, R., Loll, B., and Meinhart, A. (2008). Snapshots of the RNA Processing Factor SCAF8 Bound to Different Phosphorylated Forms of the Carboxyl-terminal Domain of RNA Polymerase II. J. Biol. Chem. 283, 22659–22669. Jasnovidova, O., and Stefl, R. (2013). The CTD code of RNA polymerase II: a structural view. Wiley Interdisciplinary Reviews: RNA 4, 1–16. Kubicek, K., Cerna, H., Holub, P., Pasulka, J., Hrossova, D., Loehr, F., Hofr, C., Vanacova, S., and Stefl, R. (2012). Serine phosphorylation and proline isomerization in RNAP II CTD control recruitment of Nrd1. Genes Dev. 26, 1891–1896. Lunde, B.M., Reichow, S.L., Kim, M., Suh, H., Leeper, T.C., Yang, F., Mutschler, H., Buratowski, S., Meinhart, A., and Varani, G. (2010). Cooperative interaction of transcription termination factors with the RNA polymerase II C-terminal domain. Nat Struct Mol Biol 17, 1195–1201. Meinhart, A., and Cramer, P. (2004). Recognition of RNA polymerase II carboxy-terminal domain by RNA-processing factors. Nature 430, 223. Ni, Z., Olsen, J.B., Guo, X., Zhong, G., Ruan, E.D., Marcon, E., Young, P., Guo, H., Li, J., Moffat, J., et al. (2011). Control of the RNA polymerase II phosphorylation state in promoter regions by CTD interaction domain-containing proteins RPRD1A and RPRD1B. Transcription 2, 237–242. Vasiljeva, L., Kim, M., Mutschler, H., Buratowski, S., and Meinhart, A. (2008). The Nrd1–Nab3–Sen1 termination complex interacts with the Ser5-phosphorylated RNA polymerase II C-terminal domain. Nat Struct Mol Biol 15, 795–804.