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# Accumulation of hns mutations specifically in stationary phase in an E. coli strain carrying an impaired rpoS locus

Desai, Stuti K and Mahadevan, S (2006) Accumulation of hns mutations specifically in stationary phase in an E. coli strain carrying an impaired rpoS locus. In: Journal of Genetics, 85 (3). pp. 221-224.

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Interplay between regulators in a cell can lead to differential gene regulation and allow the organism to respond to a range of stimuli. In bacteria, this is exemplified by the global regulators RpoS and H-NS. RpoS, the $\sigma^s$ subunit of RNA polymerase, acts as a master regulator of a subset of stationary-phase-specific genes (for a review see Loewen and Hengge-Aronis 1994). The genes under the rpoS regulon are specifically induced when cells encounter stressful conditions for growth such as nutrient deprivation experienced during stationary phase. H-NS is an abundant nucleoid structuring protein, which has been classified as a global transcriptional silencer. H-NS preferentially binds to DNA that has an intrinsic curvature and represses many genes that are required to tide over environmental stress (Goransson et al. 1990; Owen-Hughes et al. 1992). H-NS also regulates RpoS at the posttranscriptional level (Yamashino et al. 1995). The opposing actions of these two global regulators in the bacterial cell help in the homeostatic control of the genes of the rpoS regulon. Many hns strains also carry spontaneously arising second-site mutations in the rpoS gene (Barth et al. 1995). These may be selected to prevent the constitutive expression of the genes of the rpoS regulon that may be detrimental to the cell. This is consistent with the report that hns rpoS double mutants have a faster doubling time than rpoS single mutants (Barth et al. 1995).