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The Effects of Molecular Noise and Size Control on Variability in the Budding
Yeast Cell Cycle
Talia et al, Nature, 23 August 2007
William Morejón
Kelly Drinkwater
Recall the Yeast Cell Cycle
Yeast cells will grow like any other cell during the cell cycle Undergo Budding Results in a Mother
and a Daughter
This paper is solely concerned with factors governing G1 transition to S phase in Saccharomyces Cerevisiae
G1D
M
Variation in Length of G1
Size-dependent regulation Cells have differential birth size (Mbirth)
Cells will exit G1 once they have reached a certain size (Mbud)
Size-independent, fixed-time-length regulation
Molecular noise Experimental Goal:
How much of G1 Variability is due to noise?
G1 Phase
Measuring the Size Factor
Protein Based Marker for cell Size DsRed Red Fluorescent Protein under constitutive promoter
(ACT1 from actin gene) Total Red Fluorescence per cell reflects total cell protein
content
Found Exponential Growth For small Mbirth, TG1 is longer
For large Mbirth, TG1 is smaller *Implies size factor
construct
Mbud=MbirtheαT(G1
)
α=growth rate
Measuring the Time Factor
If cycle timing controlled by noisy gene expression, then N x ploidy reduces variability Used haploid, diploid and tetraploid (next slide) Strains carry Myo1-GFP
Forms a ring @ new bud neck Disappears at Cytokinesis *Allows us to measure G1
time
Ploidy and Noise in G1
Increased Ploidy reduces noise WT Haploid-some noise WT Diploid-less noise WT Tetraploids
least noise
Variability decreased by sqrt(2) for each increase in ploidy
Noise may be due to small variability in numbers of regulator molecules
Size-dependent timing in smaller cells
Relate birth size to G1 duration
Small daughter cells show strong dependency; others weak
Whi5 acts as a gatekeeper
In the nucleus, Whi5 inhibits DNA replication & budding proteins
Cyclins drive it to the cytoplasm, releasing inhibition
Divide G1 into parts T1 and T2
Size control occurs in T1
T1 varies with cell size just like overall G1 (two-slope model); very short in mothers
T2 is independent of cell size, same in mothers & daughters
Contributions
Separated variation in G1 length due to cell size, fixed-duration steps, and molecular noise
Demonstrated role of molecular noise due to 1/sqrt(2) change with ploidy doubling
Proposed two-step model of yeast G1 phase, with size- and time-regulated steps
What Bioengineers Can Use
Use ploidy or copy number as a noise control mechanism in engineered networks
Better control of replication in engineered yeast populations
Better understanding of replication possibly applicable to other organisms
Methods: Time Lapse Microscopy
Cells were imaged every 3 min on a fluorescent microscope Leica DMIRE2 inverted microscope with a Ludl motorized XY stage
Budding was scored visually by the appearance of Myo1–GFP at the incipient bud neck, and division by its disappearance, generally with single-frame accuracy.
Time-lapse microscopy data were analysed with custom software written in MATLAB