The sexual cell cycle initiation is regulated by CDK19/CYC9 in Tetrahymena thermophila

Abstract

To investigate the mechanisms underlying initiation of the sexual cell cycle in eukaryotes, we have focused on cyclins and cyclin-dependent kinases (CDKs) in the well-studied model ciliate, Tetrahyhymena thermophila. We identified two genes, CDK19 and CYC9, which are highly co-expressed with the mating-associated factors, MTA, MTB, and HAP2. Both CDK19 and CYC9 were found to be essential for mating in T. thermophila. Subcellular localization experiments suggested these proteins are located at the oral area, including the conjugation junction area, and that CDK19 or CYC9 knockout prevents mating. We found that CDK19 and CYC9 form a complex and also identified several additional subunits, which may have regulatory or constitutive functions. RNA-Seq analyses and cytological experiments showed that mating is abnormal both in ΔCDK19 and ΔCYC9, mainly at the entry to the costimulation stage. These results indicate that the CDK19/CYC9 complex initiates the sexual cell cycle in T. thermophila.