first cyclin box found in Schizosaccharomyces pombe cyclin C homolog 1 (pch1) and similar proteins
Cyclin pch1 is essential for progression through the whole cell cycle. It is a homolog of cyclin T; it forms a heterodimer with its partner kinase, cyclin-dependent kinase 9 (CDK9), that can phosphorylate both the pol II C-terminal domain (CTD) and the CTD of transcription elongation factor Spt5. Yeast Cdk9/Pch1, with mRNA capping enzyme Pct1, may also form an elongation checkpoint for mRNA quality control. Members of this family contain two cyclin boxes. This model corresponds to the first one. The cyclin box is a protein binding domain.