The Chlamydomonas cell cycle is regulated by a light/dark-responsive cell-cycle switch

Area of Science:

• Cell Biology
• Algal Physiology
• Chronobiology

Background:

• Chlamydomonas reinhardii cell division can be synchronized using light/dark cycles under various growth conditions.
• Synchronized cultures maintain synchrony for one cell cycle when shifted from mixotrophic to heterotrophic conditions.

Purpose of the Study:

• To investigate the differential effects of light exposure timing on cell growth and division in Chlamydomonas reinhardii.
• To determine if light's influence on cell division is mediated by energy metabolism or a direct cell-cycle switch.

Main Methods:

• Synchronization of Chlamydomonas reinhardii cultures under photoautotrophic and mixotrophic conditions.
• Transfer of synchronized cultures to heterotrophic conditions and exposure to light at different growth phases.
• Use of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) to inhibit photosystem II.

Main Results:

• Early light exposure accelerated cell division, while late exposure delayed it.
• Light-induced delay in cell division persisted even with photosystem II inhibition (DCMU).
• A light/dark-responsive cell-cycle switch, not energy metabolism, directly influences cell division.

Conclusions:

• The light/dark-responsive cell-cycle switch regulates the G1 to S-phase transition in Chlamydomonas reinhardii.
• This mechanism is active in cells committed to division but before reaching maximal cell mass.
• Light acts as a direct regulator of the cell cycle, overriding purely metabolic controls.