EFFECTS OF NUTRITION AND GROWTH-RATE ON LRP LEVELS IN ESCHERICHIA-COLI

Lrp (leucine-responsive regulatory protein) activates some Escherichia coli operons that function in anabolism and represses others involved in catabolism (for a review, see J. M. Calvo and R. G. Matthews, Micr obiol. Rev, 58: 466-490, 1994), This overall pattern suggests that Lrp may help cells adapt to changes in the nutritional environment, Here, we tested the idea that the nutritional richness of the medium determ ines the amount of Lrp in cells, Lrp was measured directly by Western blotting (immunoblotting) in cells grown in a chemically defined rich medium or in a minimal medium, In addition, transcription from the lrp promoter was assessed with a lacZ reporter gent, The results with the se two different measurements were nearly the same, indicating that un der the conditions employed, beta-galactosidase measurements can accur ately reflect Lrp levels, For cells in a minimal medium, Lrp levels we re consistently lowest during the logarithmic phase of grow th, but ov erall, there was not much variation in levels as a function of growth phase (1.3-fold difference between highest and lowest values), However , for cells in a rich medium, Lrp levels dropped 3- to 4-fold during t he lag phase, remained constant during the log phase, and then rose to starting levels upon entry into the stationary phase, When cells in t he log phase were compared, Lrp levels were 3- to 4-fold higher in cel ls growing in a minimal medium than those in a rich medium, The levels of lrp expression were the same or slightly higher in strains contain ing mutations in rpoS, cya, or crp compared with wild-type strains, su ggesting that neither RpoS nor the cyclic AMP (cAMP) receptor protein- cAMP complex is required for expression, On the other hand, lrp expres sion was severely restricted in cells that could not make ppGpp becaus e of mutations in relA and spoT, The reduced expression of lrp during logarithmic growth in a rich medium may be due to low ppGpp levels und er these conditions, The repressive effects of rich medium and the sti mulatory effects of ppGpp were also observed with a construct having o nly a minimal lrp promoter (-57 to +21), The results of other experime nts suggest that Lrp levels vary inversely with the growth rate of cel ls instead of being determined by some component of the medium.