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Different Levels of Catabolite Repression Optimize Growth in Stable and Variable Environments

Figure 6

Mutants isolated after repeated cycling between glucose and maltose show altered catabolite repression of the MAL genes.

(A) Flow cytometric analysis of four characteristic phenotypes that emerged after repeated glucose-to-maltose selection cycles show different degrees of glucose repression of the MAL genes. The different evolved strains with fluorescently tagged MALS genes were pregrown in maltose and then transferred to glucose media with or without maltose for 20 h of exponential growth, after which fluorescence intensities were measured using flow cytometry. (B) MAL expression in the evolved strains depends on the history of the cells. Initial state (black traces) of samples grown in maltose (left) or glucose (right) are shown, and in blue and red are the same cultures' expression levels after 20 h of growth in glucose alone or in a mixture of glucose and maltose. Note that the MAL expression level in the glucose/maltose mixture clearly depends on the history of the cells; the ancestral strain does not display this hysteresis.

Figure 6