Differential protease-mediated turnover of H-NS and StpA revealed by a mutation altering protein stability and stationary-phase survival of Escherichia coli

The Escherichia coli proteins B-NS is recognized as an important component among the major nucleoid-associated proteins, In studies off. coli strains with defects in H-NS, we discovered a mutant that phenotypically restored s tationary-phase viability (Rsv) of such strains. The Rsv phenotype was the result of a mutation that led to severalfold higher levels of the functiona lly and structurally related StpA protein. This mutation was a base pair ch ange in the stpA structural gene, and the amino acid substitution in the St pA protein altered its turnover properties, suggesting a role for this resi due in a cleavage site for proteolysis, We determined the stability of the StpA and the II-NS proteins and found that the StpA protein was degraded re latively rapidly in strains lacking functional N-NS, whereas H-NS remained stable irrespective of the presence/absence of StpA, Using protease-deficie nt mutants, we obtained evidence that the Lon protease was responsible fur the degradation of StpA. The differential turnover of the nucleoid-associat ed proteins is suggested to contribute to the regulation of their stoichiom etry and ratio in terms of homo- and heteromer formation. We conclude that StpA, in contrast to H-NS, is present mainly in heteromeric form in E. coli .