GROWTH-PHASE VARIATION OF INTEGRATION HOST FACTOR LEVEL IN ESCHERICHIA-COLI

We have measured the intracellular abundance of integration host facto r (IHF), a site-specific, heterodimeric DNA-binding protein, in expone ntial- and stationary-phase cultures of Escherichia coli K-12. Western immunoblot analysis showed that cultures that had been growing expone ntially for several generations contained 0.5 to 1.0 ng of IHF subunit s per mu g of total protein and that this increased to 5 to 6 ng/mu g in late-stationary-phase cultures. IHF is about one-third to one-half as abundant in exponentially growing cells as MU, a structurally relat ed protein that binds DNA with little or no site specificity. Wild-typ e IHF is metabolically stable, but deletion mutations that eliminated one subunit reduced the abundance of the other when cells enter statio nary phase. We attribute this reduction to the loss of stabilizing int eractions between subunits. A mutation that inactivates IHF function b ut not subunit interaction increased IHF abundance, consistent with re sults of previous work showing that IHF synthesis is negatively autore gulated. We estimate that steady-state exponential-phase cultures cont ain about 8,500 to 17,000 IHF dimers per cell, a surprisingly large nu mber for a site-specific DNA-binding protein with a limited number of specific sites. Nevertheless, small reductions in IHF abundance had si gnificant effects on several IHF-dependent functions, suggesting that the wild-type exponential phase level is not in large excess of the mi nimum required for occupancy of physiologically important IHF-binding sites.