The assimilation of amino acids by bacteria. 20. The incorporation of labelled amino acids by disrupted staphylococcal cells

Abstract

A sedimentable fraction, consisting of disrupted cells, capable of incorporating a number of C14-labelled amino acids, was prepared from suspensions of Staphylococcus aureus after exposure in a sucrose medium to supersonic vibration. Incorporation of C14-glutamic acid requires a source of energy which may be adenosine triphosphate (ATP) and hexose diphosphate (HDP). When glutamic acid is the only amino acid added to the incubation mixture, incorporation ceases after only a fraction of the added amino acid has been incorporated. When the concentration of added amino acid is low, the incorporation ceases at a level dependent upon that concentration. Preparations rendered radioactive by incorporation of C14-glutamic acid lose radioactivity if subsequently incubated with C12-glutamic acid and ATP + HDP but not with C12-glutamic acid alone, ATP + HDP alone, or C12-glycine or C12-aspartic acid and ATP + HDP. In the presence of a complete mixture of amino acids, incorporation of C14-glutamic acid proceeds linearly for some hours and is accompanied by a significant increase in protein-N. Depletion of the nucleic acid content of the disrupted cell preparation results in marked decrease in the rate and amount of incorporation. Incorporation can be restored in such preparations by addition of staphylococcal nucleic acid, desoxyribonucleic acid being more active than ribonucleic acid on a dry weight basis. The degree of in-activation of incorporation, and subsequent activation by nucleic acids, depends upon (a) the time of exposure of the cells to supersonic vibration during disintegration and (b) the amino acid incorporated. Incorporation is inhibited by chloramphenicol, bacitracin and penicillin but the sensitivity varies with different amino acids. In all cases studied, inhibition ceases at a plateau value which again varies with the individual amino acid concerned.