Abstract
Although the decision to proceed through cell division depends largely on the metabolic status or the size of the cell, the timing of cell division is often set by internal clocks such as the circadian clock. Light is a major cue for circadian clock entrainment, and for photosynthetic organisms it is also the main source of energy supporting cell growth prior to cell division. Little is known about how light signals are integrated in the control of S phase entry. Here, we present an integrated study of light-dependent regulation of cell division in the marine green alga Ostreococcus. During early G1, the main genes of cell division were transcribed independently of the amount of light, and the timing of S phase did not occur prior to 6 hours after dawn. In contrast S phase commitment and the translation of a G1 A-type cyclin were dependent on the amount of light in a cAMP–dependent manner. CyclinA was shown to interact with the Retinoblastoma (Rb) protein during S phase. Down-regulating Rb bypassed the requirement for CyclinA and cAMP without altering the timing of S phase. Overexpression of CyclinA overrode the cAMP–dependent control of S phase entry and led to early cell division. Therefore, the Rb pathway appears to integrate light signals in the control of S phase entry in Ostreococcus, though differential transcriptional and posttranscriptional regulations of a G1 A-type cyclin. Furthermore, commitment to S phase depends on a cAMP pathway, which regulates the synthesis of CyclinA. We discuss the relative involvements of the metabolic and time/clock signals in the photoperiodic control of cell division.
Highlights
The cell division cycle (CDC) is a highly conserved and regulated process, which controls the proliferation of unicellular organisms and development and tissue renewal in multicellular organisms
The transcriptional regulation of cell cycle progression in S phase is controlled in mammals and plants by the E2F transcription factors and these are sequestrated by the Retinoblastoma protein (Rb)
Light is a source of energy for photosynthesis, that is required for cell growth prior to cell division but it can act as a signal controlling the timing of cell cycle events
Summary
The cell division cycle (CDC) is a highly conserved and regulated process, which controls the proliferation of unicellular organisms and development and tissue renewal in multicellular organisms. The transcriptional regulation of cell cycle progression in S phase is controlled in mammals and plants by the E2F transcription factors and these are sequestrated by the Retinoblastoma protein (Rb). In budding yeast, it is controlled by the transcription factor Swi4/6-dependent cell cycle box-Binding Factor (SBF) which is sequestrated by Whi. On phosphorylation of Rb by G1 Cyclin/CDC complexes, such as CyclinD-Cdk, E2F transcription factors are released leading to S phase commitment. In yeast on phosphorylation of Whi, by Cln3/Cdc, SBF is released leading to S phase commitment. Rb and Whi respectively are critical players in linking cell size or metabolic status to cell cycle progression [8,9]
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