Summary
Cell size and biosynthesis are inextricably linked. As cells grow, total protein synthesis increases in proportion to cell size so that protein concentrations remain constant. As an exception, the budding yeast cell-cycle inhibitor Whi5 is synthesized in a constant amount per cell cycle, so that it is diluted in large cells to trigger division. Here, we show that this size-independent expression of Whi5 results from size-independent transcription. A screen for similar genes identified histones as the major class of size-independent transcripts during the cell cycle, consistent with histone synthesis being coupled to genome content rather than cell size. However, during asymmetric division size-independent transcription is insufficient for size-independent protein expression and chromatin-binding ensures equal amounts of protein are partitioned to unequally sized cells to maintain size-independent protein amounts. Thus, specific transcriptional and partitioning mechanisms determine size-independent protein expression to control cell size.
Competing Interest Statement
The authors have declared no competing interest.