Heldt, F. S. | bioRxiv Science

bioRxiv2019

A single light-responsive sizer can control multiple-fission cycles in Chlamydomonas

Proliferating cells need to coordinate cell division and growth to maintain size homeostasis. Any systematic deviation from a balance between growth and division results in progressive changes of cell size over subsequent generations. While most eukaryotic cells execute binary division after a mass doubling, the photosynthetic green alga Chlamydomonas can grow more than eight-fold during daytime before undergoing rapid cycles of DNA replication, mitosis and cell division at night, which produce up to 16 daughter cells. Here, we propose a mechanistic model for multiple fission and size control in Chlamydomonas. The model comprises a light-sensitive and size-dependent biochemical toggle switch that acts as a sizer and guards transitions into and exit from a phase of cell-division cycle oscillations. We show that this simple sizer-oscillator arrangement reproduces the experimentally observed features of multiple-fission cycles and the response of Chlamydomonas cells to different light-dark regimes. Our model also makes testable predictions about the dynamical properties of the biochemical network that controls these features and about the networks makeup. Collectively, these results provide a new perspective on the concept of a commitment point during the growth of Chlamydomonas cells and hint at an intriguing continuity of cell-size control in different eukaryotic lineages.\n\nGraphical abstract\n\nO_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=182 SRC=\"FIGDIR/small/648436v1_ufig1.gif\" ALT=\"Figure 1\">\nView larger version (35K):\norg.highwire.dtl.DTLVardef@48f9corg.highwire.dtl.DTLVardef@170625eorg.highwire.dtl.DTLVardef@14a20a0org.highwire.dtl.DTLVardef@1a3ff9a_HPS_FORMAT_FIGEXP M_FIG C_FIG O_LIG1-sizer and S/M-oscillator can give rise to multiple-fission cycles in Chlamydomonas\nC_LIO_LILight-responsive bistable switch may guard transition between G1 and S/M-cycles\nC_LIO_LIIllumination increases S/M-entry threshold, causing multiple-fission cycles\nC_LIO_LIDark shift lowers S/M-entry threshold, allowing small cells to commit to fewer divisions\nC_LI

cell biology↗

bioRxiv2018

Dilution and titration of cell-cycle regulators may control cell size in budding yeast

The size of a cell sets the scale for all biochemical processes within it, thereby affecting cellular fitness and survival. Hence, cell size needs to be kept within certain limits and relatively constant over multiple generations. However, how cells measure their size and use this information to regulate growth and division remains controversial. Here, we present two mechanistic mathematical models of the budding yeast (S. cerevisiae) cell cycle to investigate competing hypotheses on size control: inhibitor dilution and titration of nuclear sites. Our results suggest that an inhibitor-dilution mechanism, in which cell growth dilutes the transcriptional inhibitor Whi5 against the constant activator Cln3, can facilitate size homeostasis. This is achieved by utilising a positive feedback loop to establish a fixed size threshold for the SO_SCPCAPTARTC_SCPCAP transition, which efficiently couples cell growth to cell cycle progression. Yet, we show that inhibitor dilution cannot reproduce the size of mutants that alter the cells overall ploidy and WHI5 gene copy number. By contrast, size control through titration of Cln3 against a constant number of genomic binding sites for the transcription factor SBF recapitulates both size homeostasis and the size of these mutant strains. Moreover, this model produces an imperfect sizer behaviour in G1 and a timer in S/G2/M, which combine to yield an adder over the whole cell cycle; an observation recently made in experiments. Hence, our model connects these phenomenological data with the molecular details of the cell cycle, providing a systems-level perspective of budding yeast size control.

systems biology↗