The non-canonical CTD of RNAP-II is essential for productive RNA synthesis in Trypanosoma brucei.

Abstract

The carboxy-terminal domain (CTD) of the largest subunit (RPB1) of RNA polymerase II (RNAP-II) is essential for gene expression in metazoa and yeast. The canonical CTD is characterized by heptapeptide repeats. Differential phosphorylation of canonical CTD orchestrates transcriptional and co-transcriptional maturation of mRNA and snRNA. Many organisms, including trypanosomes, lack a canonical CTD. In these organisms, the CTD is called a non-canonical CTD or pseudo-CTD (PsiCTD. In the African trypanosome, Trypanosoma brucei, the PsiCTD is approximately 285 amino acids long, rich in serines and prolines, and phosphorylated. We report that T. brucei RNAP-II lacking the entire PsiCTD or containing only a 95-amino-acid-long PsiCTD failed to support cell viability. In contrast, RNAP-II with a 186-amino-acid-long PsiCTD maintained cellular growth. RNAP-II with PsiCTD truncations resulted in abortive initiation of transcription. These data establish that non-canonical CTDs play an important role in gene expression.