BIMODAL ACTION OF SPERMINE ON RIBOSOMAL PEPTIDYLTRANSFERASE AT LOW CONCENTRATION OF MAGNESIUM-IONS

At 6 mM Mg2+, submillimolar concentrations of spermine affect the end- point as well as the kinetic phase of puromycin reaction in a cell-fre e system from Escherichia coli. When the ternary complex AcPhe-tRNA-po ly(U)-ribosome (complex C) is formed in the absence of ribosomal wash (FWR fraction), the final degree of AcPhe-puromycin synthesis is raise d from 12% to 60%, as the concentration of spermine increases from zer o to 200 mu M, However, spermine displays partial noncompetitive inhib ition at the kinetic phase of the reaction. The inhibitory effect of s permine is related with its binding to AcPhe-tRNA. When complex C is f ormed in the presence of FWR fraction, spermine slightly affects the f inal degree of puromycin reaction. However, AcPhe-puromycin synthesis is markedly stimulated by the addition of relatively low concentration s of spermine. Kinetic analysis of the activation phase revealed that spermine attached on a specific site of complex C, acts as a nonessent ial, partial noncompetitive activator. The stimulatory effect of sperm ine seems to be due to its interaction with ribosomes. Further additio ns of spermine cause partial noncompetitive inhibition on the puromyci n reaction. This result suggests that complex C possesses a second bin ding site, responsible for the inhibitory effect of spermine. Both act ivator and inhibitor sites can be occupied by spermine at the same tim e.