Isolation of divergent Toxoplasma cyclins via yeast two hybrid.  At different stages of development apicomplexan parasites change the scale if not their mode of replication.  Simple binary division characterizes the tachyzoite of Toxoplasma, but during the definite cycle in the cat merozoites replicate by the more common multinuclear process called schizogony.  The flexibility in choosing a cell cycle suggests the apicomplexan cyclin pathways may be fairly unique.  We recently adopted a yeast two-hybrid strategy to isolate these proteins.  From independent yeast screens, we identified two divergent cyclins based on their functional interaction with a previously described cyclin–dependent kinase from Toxoplasma.  TgCYC1 shares the highest similarity with the cyclin H family, while TgCYC2 falls within the A/B family of cyclins  (Kvaal et al., 2001).  Three additional cyclins have emerged from the Plasmodium and Cryptosporidium genome databases (five all total).  As the efforts to sequence the Toxoplasma genome get underway, it is likely that additional cyclins will be found.