Kinetics of Biosynthesis of Iron-Regulated Membrane Proteins in Escherichia coli

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Abstract
Using biological Fe chelators to specifically control Fe availability to E. coli K-12 in conjunction with radioactive pulse-labels, the biosynthesis of 6 Fe regulated membrane proteins was examined. Fe deprivation induced the synthesis of 5 proteins, which had MW of 83,000 (83K), 81K (Fep.), 78K (TonA), 74K (Cir) and 25K. The kinetics of induction were the same in entA and entA+ strains, but were affected by the initial Fe availability in the media. Fe poor cells induced rapidly (half-time, 10 min), whereas Fe rich cells began induction after a lag and showed a slower induction half-time (30 min). Within this general pattern of induction after Fe deprivation, several different kinetic patterns were apparent. The 83K, 81K and 74K proteins were coordinately controlled under all of the conditions examined. The 78K and 25K proteins were regulated differently. The synthesis of a previously unrecognized 90K inner membrane protein was inhibited by Fe deprivation and stimulated by Fe repletion. Both ferrichrome and ferric enterobactin completely repressed 81K and 74K synthesis when the siderophores were supplied at concentrations of 5 .mu.M in vivo (half-time, 2.5 min). At concentrations < 5 .mu.M, however, both siderophores repressed synthesis only temporarily; the duration of repression was porportional to the amount of ferric siderophore added. The half-lives of the 81K and 74K mRNA species, as measured by rifampin treatment, were 1.2 and 1.6 min, respectively. Enteric bacteria are apparently capable of instantaneously detecting and reacting to fluctuations in the extracellular Fe concentration and that they store Fe during periods of Fe repletion for utilization during periods of Fe stress. Neither Fe storage nor Fe regulation of envelope protein synthesis is dependent on the ability of the bacteria to form heme.