A REGULATOR OF THE FLAGELLAR REGULON OF ESCHERICHIA-COLI, FLHD, ALSO AFFECTS CELL-DIVISION

The role of an activator of flagellar transcription in Escherichia col i, flhD, was investigated in the regulation of cell division. When gro wn in tryptone broth, flhD mutant cells divided exponentially until th ey reached a cell density of 2.5 x 10(9) cells per ml. Wild-type cells and flhC mutant cells divided exponentially until they reached a cell density of 4 x 10(7) cells per ml. flhD mutant cells divided 5 times more than wild-type cells before they reduced their cell division rate and reached a cell density 37 times higher than that of wild-type or flhC mutant cultures. In stationary phase, the biomasses of all cultur es were similar; however, flhD mutant cells were significantly smaller . Additional tryptone, Casamino Acids, and individual amino acids, add ed at the beginning of growth, allowed wild-type cells to grow to high er cell densities. Serine was determined to have the greatest effect. In contrast, the addition of Casamino Acids did not exhibit an effect upon flhD mutant cells. flhD mutant cells exhibited normal rates of up take of serine and other amino acids. In both wild-type and flhD mutan t cultures, the concentrations of serine in the media dropped from 140 to 20 mu M within the first 2 h of growth. Serine concentrations and cell division rates were highly correlated. Wild-type cells reduced th eir cell division rate at a medium concentration of 50 mu M serine, an d the addition of serine at this time caused cells to resume a higher rate of division. We conclude that the reduction of the cell division rate in wild-type cells is caused by the depletion of serine from the medium and that flhD mutant cells seem to be unable to sense this depl etion.