THE SIZE CONTROL OF FISSION YEAST REVISITED

An analysis was made of cell length and cycle time in time-lapse films of the fission yeast Schizosaccharomyces pombe using wild-type (WT) c ells and those of various mutants, The more important conclusions abou t 'size controls' are: (1) there is a marker in G(2) in WT cells provi ded by a rate change point (RCP) where the linear rate of length growt h increases by similar to 30%, The period before this RCP is dependent on size and can be called a 'sizer', The period after the RCP is near ly independent of size and can be called a 'timer', The achievement of a critical threshold size is at or near the RCP which is on average a t about 0.3 of the cycle (halfway through G(2)) This is much earlier t han was previously believed, (2) The RCP is at about the time when H1 histone kinase activity and the B type cyclin cdc13 start to rise in p reparation for mitosis, The RCP is also associated with other metaboli c changes, (3) In wee1 mutants, the mitotic size control is replaced b y a G(1)/S size control which is as strong as the mitotic control, As in WT cells, there is a sizer which precedes the RCP followed by a tim er but the RCP is at about the G(1)/S boundary and has a larger increa se (similar to 100%) in rate, (4) cdc25 is not an essential part of th e size control at mitosis or at the G(1)/S boundary, (5) Three further situations have been examined in which the mitotic size control has b een abolished, First, induction synchronisation by block and release o f cdc2 and cdc10, In the largest oversize cells which are produced, th e RCP is pushed back to the beginning of the cycle, There is no sizer period but only a timer. Second, when both the antagonists wee1 and cd c25 are absent in the double mutant wee1-50 cdc25 Delta. In this inter esting situation there is apparently no mitotic size control and the c ycle times are quantised, Third, in rum1 Delta wee1-50 where the norma l long G(1) in wee1 is much reduced, there is probably no size control either in G(1) or in G(2) causing a continuous shortening of division length from cycle to cycle.