SUMMARY: The time course of the changes in the percentages by weight of ribonucleic acid, deoxyribonucleic acid and nucleic acid fragment (%RNA, %DNA and %Nt respectively) and of their absolute rates of synthesis in a strain of var. () has been studied throughout a complete normal growth eyete and compared with the changes which occur when penicillin G is added to the culture in the late logarithmic phase of growth. In normal cultures, the rate of cell growth (as dry weight) appears to be controlled by the %RNA, while the %Nt varies inversely with the rate of synthesis of RNA, the Nt/(RNA + DNA) weight ratio being poised at about 0·1 during rapid growth. The rate of synthesis of DNA and the %DNA, on the other hand, are related to the size of the cells in such a way as to maintain the weight of DNA per cell constant. The presence of 1 μg. of penicillin/ml. of culture at a cell dry-weight concentration of 200 μg./ml. causes an accumulation of free nucleotide+nucleoside and a depletion of free purine, and a change of the Nt/(RNA+DNA) ratio from . 0·1 to . 0·2. The absolute rate of RNA synthesis is decreased more than that of DNA, but initially both %RNA and %DNA increase because of the inhibition of synthesis of some unidentified component which contributes significantly to the cell dry weight. The synthesis of an unidentified phosphate ester (XP) is also inhibited at an early stage in the penicillin-treated cultures. At the same time as the chemical changes, there are changes in the surfaces of the penicillin-treated cells, shown by their light-scattering properties and suspension stability. An attempt has been made to integrate the new observations into the general picture of penicillin action.


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