The assimilation of amino acids by bacteria. 19. The inhibition of phenylalanine incorporation in Staphylococcus aureus by chloramphenicol and p-chlorophenylalanine.

In the previous paper of this series (Gale & Folkes, 1953c) it was shown that incubation of washed suspensions of Staphylococcus aureus (Micrococcus pyogenes var. aureus) with glucose and [14C]glutamic acid resulted in incorporation of the labelled glutamic acid into the protein fraction of the cells. This process occurred in the absence of demonstrable protein synthesis and underwent considerable modification in properties when a complete mixture ofamino acids was added. Under these conditions net synthesis of protein took place and was inhibited by bactericidal concentrations of chloramphenicol (Gale & Folkes, 1953a, b). The incorporation of labelled glutamic acid was equally sensitive to the antibiotic as long as a complete mixture of amino acids was present but became markedly less sensitive when glutamic acid was the only amino acid present in the incubation mixture. Woolley (1951) found that phenylalanine would decrease the antibacterial action ofchloramphenicol when this was present in concentrations which partially inhibited growth, although the amino acid was ineffective as antagonist when growth inhibition was complete. He pointed out that there are structural similarities between chloramphenicol and phenylalanine, and suggested that the antibiotic acts as an inhibitor of phenylalanine metabolism and may prevent the incorporation of phenylalanine into proteins. Halvorson & Spiegelman (1952) investigated the inhibition of growth and enzyme formation in yeast by amino acid analogues, and found that certain derivatives of phenylalanine, oand p-fluorophenylalanine in particular, were potent inhibitors whose action could be competitively antagonized by the corresponding amino acid only. It was thought that a study of the actions of chloramphenicol and an analogue of phenylalanine on the incorporation of that amino acid into the protein fraction of Staph. aureus mightthrow light on the relationship between amino acid incorporation and protein synthesis, and also on the mode of action of chloramphenicol. METHODS