Sexual cell cycle initiation is regulated by CDK19 and CYC9 in Tetrahymena thermophila

Abstract

To investigate the mechanisms underlying initiation of the sexual cell cycle in eukaryotes, we have focused on cyclins and cyclindependent kinases (CDKs) in the well-studied model ciliate, Tetrahymena 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 that 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 sequencing analyses and cytological experiments showed that mating is abnormal in both ΔCDK19 and ΔCYC9, mainly at the entry to the co-stimulation stage. These results indicate that the CDK19-CYC9 complex initiates the sexual cell cycle in T. thermophila.

Tetrahymena meiosis: Simple yet ingenious

“…Sexual reproduction begins when starved cells of complementary sexes meet. They first enter the so-called costimulation stage, in which they sense the presence of cells of a different mating type (see [8]). These cells can mate ("conjugate") and initiate meiosis within 2 hours and complete it within 5 hours [9].…”

“…Meiotic genes show a characteristic temporal transcription pattern, with maximal expression in mating cells occurring from as early as 2 hours after mixing meiosis-competent cells [15]. Interestingly, some meiotic genes start being expressed at the costimulation stage, prior to pairing between mating partners (see NCBI dataset GSE132677; [8]). Although at this point cells may return to the vegetative cycle if the nutritional situation improves, such preparedness will speed up the mating process if starvation persists.…”

Functional characterization of RebL1 highlights the evolutionary conservation of oncogenic activities of the RBBP4/7 orthologue inTetrahymena thermophila

“…Tetrahymena thermophila, a unicellular ciliate protozoan, is an excellent experimental system to study evolutionarily conserved chromatin-related processes, including gene expression and cell cycle regulation (55)(56)(57)(58). Tetrahymena has two structurally and functionally distinct nuclei, a germ-line diploid micronucleus (MIC) and a polyploid somatic macronucleus (MAC), that are physically separated within the same cell (59).…”

Transcriptome analysis of the binucleate ciliate Tetrahymena thermophila with asynchronous nuclear cell cycles

“…Tetrahymena's developmental and vegetative cell cycles must employ novel regulatory pathways to coordinate the fate of chromosomes in functionally distinct nuclei. Consistent with this premise, T. thermophila encodes 34 predicted cyclins, and 20 predicted cyclin-dependent kinases (CDKs) (Stover and Rice 2011, Yan, Dang et al 2016, Ma, Yan et al 2020, a subset that are only expressed in mating cells (Ma, Yan et al 2020). To gain insight into the vegetative cell cycle of Tetrahymena, we used centrifugal elutriation to profile gene expression across 1.5…”