Fig. 1.—

Fluctuating selection enriched for genomic fragments driving noisy yEGFP expression. (A) After insertion of genomic fragments from the EC1118 haploid derivative 59A into centromeric promoterless yEGFP-coding plasmids and transformation of the library in the Saccharomyces cerevisiae strain CEN-PK, fluctuating selection ensuring enrichment in noisy promoters was performed by seven rounds of cell sorting, with alternatively the 5% highest or the 5% lowest fluorescence levels conserved (except for the first round where only the fluorescent cells were sorted [around 4%]). Between each round, cells were grown overnight, diluted the next morning, and sorted in exponential phase in the afternoon. (B) Fluorescence distribution in the population containing the yEGFP-fused genomic library before and after fluctuating selection. Mean and CV values are increased after selection. (C) CV of yEGFP expression in clones from selected and control populations. The clones from the selected population (right) show higher noise level than the clones from the control population (left) (T-test, P = 0.047). Each data point represents the CV of yEGFP expression among several thousand individual cells.