Shrinking Daughters: Rlm1-Dependent G1/S Checkpoint Maintains Saccharomyces cerevisiae Daughter Cell Size and Viability

The Rlm1 transcription factor is a target of the cell wall integrity pathway. We report that an rlm1 Delta mutant grown on a nonfermentable carbon source at low osmolarity forms cell groups in which a mother cell is surrounded by smaller satellite-daughter cells. Mother cells in these groups progressed through repeated rounds of cell division with normal rates of bud growth and genetic stability; however, these cells underwent precocious START relative to wild-type mothers. Thus, once activated, Rlm1 delays the transition from G(1) to S, a mechanism we term the cell wall/START (CW/START) checkpoint. The rlm1 Delta satellite-cell phenotype is suppressed by deletion of either SLT2, which encodes the kinase that activates Rlm1, or SWI4, which is also activated by Slt2; suggesting that Slt2 can have opposing roles in regulating the START transition. Consistent with an Rlm1-dependent CW/START checkpoint, rlm1 Delta satellite daughters were unable to grow or divide further even after transfer to rich medium, but UV irradiation in G1 could partially rescue rlm1 Delta satellite daughters in the next division. Indeed, after cytokinesis, these satellite daughters shrank rapidly, displayed amorphous actin staining, and became more permeable. As a working hypothesis, we propose that duplication of an actin-organizing center in late G(1) may be required both to progress through START and to reestablish the actin cytoskeleton in daughter cells.