Genetically identical cells in the same environment can exhibit different phenotypes, and a single cell can switch between distinct phenotypes in a stochastic manner. In the classic example of lactose metabolism in E. coli, the lac genes are fully expressed for every cell in a population under high extracellular concentrations of inducers, such as the lactose analog methyl- b -D-thiogalactoside (TMG). However, at moderate inducer concentrations, the lac genes are highly expressed in only a fraction of a population, which may confer a fitness advantage for the entire population. We investigate the molecular mechanism that controls the stochastic phenotype switching of a single cell. Lactose metabolism is controlled by the lac operon, which consists of the lacZ , lacY , and lacA genes encoding beta-galactosidase, lactose permease, and transacetylase, respectively. Expression of the operon is regulated by the lactose repressor, which dissociates from its specific binding sequences of DNA, the lac operators, in the presence of inducer to allow transcription. The production of the permease increases inducer influx , resulting in positive feedback on expression of the lac operon. We label the permease with a yellow fluorescent protein. Above a certain threshold of permease numbers, a cell will have a fluorescent membrane and be capable of lactose metabolism, but below this threshold, a cell will be nonfluorescent and be incapable of lactose metabolism.
			This time-lapse movie shows fluorescence overlay (yellow) on phase contrast images of a cell dividing into two genetically identical daughter cells. One daughter cell changes into a phenotype with very high fluorescence, while the other daughter cell does not. This change in phenotype is the result of the stochastic, full dissociation of the tetrameric repressor from all of its binding sites.
	The tetrameric repressor can simultaneously bind to two operators to form a DNA loop. Under low inducer concentrations, the repressor cannot be pulled off the DNA by the inducer. Rather, spontaneous, partial dissociations of the repressor result in transcription of one mRNA and a small burst of proteins. However, infrequent events of complete dissociation of the repressor result in large bursts of permease expression that trigger induction of the lac operon. Thus, we show that a stochastic, single molecular event, the complete dissociation of the tetrameric repressor, determines a cell's phenotype and show that DNA looping is a crucial molecular component of this bistable genetic switch. Reference Choi Paul, Cai Long, Frieda Kirsten, Xie Xiaoliang Sunney(2008). A stochastic single-molecule event triggers phenotype switching of a bacterial cell. Science . 322 : 442-446. Return To Top - Go Back to the Previous Page