DISSOCIATION RATE OF COGNATE PEPTIDYL-TRANSFER-RNA FROM THE A-SITE OF HYPER-ACCURATE AND ERROR-PRONE RIBOSOMES

The binding stability of the aminoacyl-tRNA site (A-site), estimated from the dissociation rate constant k(d), of AcPhe-Phe-tRNA(Phe) has been studied for wild-type (wt), for hyperaccurate ribosomes altered in S12 [streptomycin-dependent (SmD) and streptomycin-pseudodependent (SmP) phenotypes], for error-prone ribosomes altered in S4 (Ram phenotype), and for ribosomes in complex with the error-inducing aminoglycosides streptomycin and neomycin. The AcPhe(2)-tRNA stability is slightly and identically reduced for SmD and SmP phenotypes in relation to wt ribosomes. The stability is increased (k(d) is reduced) for Ram ribosomes to about the same extent as the proofreading accuracy is decreased for this phenotype. k(d) is also reduced by the action of streptomycin and neomycin, but much less than the reduction in proof-reading accuracy induced by streptomycin. Similar k(d) values for SmD and SmP ribosomes indicate that the cause of streptomycin dependence is not excessive drop-off of peptidyl-tRNAs from the A-site.