A gene from the hypotrichous ciliate Stylonychia lemnae coding for a protein with homology to cyclin B

Two distinct classes of mitotic cyclin homologues, Cyc1 and Cyc2, are involved in cell cycle regulation in the ciliate Paramecium tetraurelia

The eukaryotic cell cycle is regulated by the sequential activation of different CDK/cyclin complexes. Two distinct classes of mitotic cyclin homologues, CYC1 and CYC2, have been identified and cloned for the first time in the ciliate Paramecium. Cyc1 is 324 amino acids long with a predicted molecular mass of 38 kDa, whereas Cyc2 is 336 amino acids long with a predicted molecular mass of 40 kDa. They display 42-51% sequence identity to other eukaryotic mitotic cyclins within the 'cyclin box' region. The conserved 'cyclin box' and 'destruction box' elements can be identified within each of the sequences. Genomic Southern blot analysis indicated that the CYC1 gene has two isoforms, with 92.3% and 85.9% identify at the amino acid level and at the nucleotide level, respectively. Both Cyc1 and Cyc2 proteins showed characteristic patterns of accumulation and destruction during the vegetative cell cycle, with Cyc1 peaking at the point of commitment to division (PCD), and Cyc2 reaching the maximal level late in the cell cycle. Immunoprecipitation experiments with antibodies specific to Cyc1 and Cyc2 indicated that Cyc1 and Cyc2 associate with distinct CDK homologues. Both immunoprecipitates exhibited histone H1 kinase activity that oscillated in the cell cycle in parallel with the respective amount of cyclins present. Histone H1 kinase activity associated with Cyc1 reached a peak at PCD while Cyc2 showed maximal activity when about 75% cells have completed cytokinesis. We propose that Cyc1 may be involved in commitment to division, in association with the CDK that binds to p13suc1, Cdk3, and that the Cyc2/Cdk2 complex may regulate cytokinesis. PCR-amplification revealed similar sequences in Tetrahymena, Sterkiella, Colpoda and Blepharisma, suggesting the conservation of the cyclin genes within ciliates. Although cell cycle regulation in ciliates differs in some respects from that of other eukaryotes, the cyclin motifs have clearly been conserved during evolution.

Commitment to division in ciliate cell cycles

Near the end of the cell cycle, ciliates commit irreversibly to cell division. The point of commitment occurs at the time of oral polykinetid assembly and micronuclear anaphase. The commitment is a checkpoint which requisites a threshold cell mass/DNA ratio and stomatogenesis. It is also a physiological transition point, involving cdk protein kinases similar to those of other eukaryotes. Both P34 kD and P36 kD kinases, similar to the S. pombe cdc2 kinases, have been described to have activity as monomers. Subsequent to commitment to division, dramatic cytoskeletal modifications occur for separation of organelles, cortex morphogenesis and cytokinesis. Numerous mutants affecting cytoskeletal function associated with the division process have been obtained in several species. Of these, only the cc1 mutant in Paramecium affects cell cycle progression prior to commitment to division. The material reviewed is used to speculate about the mechanisms of regulation of pre-fission morphogenesis and cell division related processes in ciliates.

Macronuclear gene-sized molecules of hypotrichs

The macronuclear genome of hypotrichous ciliates consists of DNA molecules of gene-sized length. A macronuclear DNA molecule contains a single coding region. We have analyzed the many hypotrich macronuclear DNA sequences sequenced by us and others. No highly conserved promoter sequences nor replication initiation sequences have been identified in the 5' nor in the 3' non-translated regions, suggesting that promoter function in hypotrichs may differ from other eukaryotes. The macronuclear genes are intron-poor; approximately 19% of the genes sequenced to date have one to three introns. Not all macronuclear DNA molecules may be transcribed; some macronuclear molecules may not have any coding function. Codon bias in hypotrichs is different in many respects from other ciliates and from other eukaryotes.