Variation in HU composition during growth of Escherichia coli: the heterodimer is required for long term survival.

The histone-like dimeric HU protein of Escherichia coli is encoded by two closely related genes, hupA and hupB. We show here that expression from the single hupA promoter and from the three hupB promoters varies during growth phase. The weak hupB-P4 promoter is active immediately after dilution. Transcription of the hupA gene is activated early in logarithmic phase. A little later, at mid to late exponential phase, RNA originating at the hupB-P2 promoter is detected. The hupB-P3 promoter is activated last when the cells enter stationary phase. Although the hup mRNAs are unstable, the HU protein is very stable so that the variations in the mRNAs synthesis are reflected in the level of the two HU subunits and in the composition of HU dimers. Cells growing exponentially contain a mixture of homodimeric alpha 2 and heterodimeric alpha beta but no beta 2 is detected. In stationary cells, the predominant form is the heterodimer alpha beta. The presence of the heterodimeric form is required for optimal survival of E. coli after prolonged starvation. The three forms of HU are not equivalent, since beta 2 is incapable of promoting formation of DNA supercoiling like alpha beta and alpha 2 do. The putative roles of each form of HU are discussed.