DIURNAL CELL DIVISION REGULATED BY GATING THE G1 /S TRANSITION IN ENTEROMORPHA COMPRESSA (CHLOROPHYTA)(1).

The cell-cycle progression of Enteromorpha compressa (L.) Nees (=Ulva compressa L.) was diurnally regulated by gating the G1 /S transition. When the gate was open, the cells were able to divide if they had attained a sufficient size. However, the cells were not able to divide while the gate was closed, even if the cells had attained sufficient size. The diurnal rhythm of cell division immediately disappeared when the thalli were transferred to continuous light or darkness. When the thalli were transferred to a shifted photoperiod, the rhythm of cell division immediately and accurately synchronized with the shifted photoperiod. These data support a gating-system model regulated by light:dark (L:D) cycles rather than an endogenous circadian clock. A dark phase of 6 h or longer was essential for gate closing, and a light phase of 14 h was required to renew cell division after a dark phase of >6 h.