Figure 9.

Proposed Model for Chlamydomonas Cell Cycle Regulation by CYCA1-CDKA1 and CYCB1-CDKB1.

We hypothesize that the main in vivo cyclin-CDK complexes are CYCA1-CDKA1 and CYCB1-CDKB1. The APC negatively regulates CYCA1-CDKA1 and CYCB1-CDKB1; CYCB1-CDKB1 also negatively regulates CYCA1-CDKA1 (red bars). We propose CYCA1 and CYCB1 as predominant, specific in vivo activators of CDKA1 and CDKB1, respectively (heavy green arrows). CDKA1 activation of CYCA1 transcription represents a possible positive feedback loop (circled +); CDKA1 activation of CYCB1-CDKB1 with ensuing repression of CYCA1-CDKA1 by CYCB1-CDKB1 represents a possible negative feedback loop (circled −). Blue arrows, promotion of cell division cycle events; blue bars, inhibition of cell division cycle events; green arrows, activation of control circuitry; red bars, inhibition of control circuitry; dashed green line, possible activation of APC by CYCB1-CDKB1.