Early studies demonstrated roles for ribosomal protein L3 in peptidyltransferase center formation and the ability of cells to propagate viruses. More recent studies have linked these two processes via the effects of mutants and drugs on programmed -1 ribosomal frameshifting. Here, we show that mutant forms of L3 result in ribosomes having increased affinities for both aminoacyl- and peptidyl-tRNAs. These defects potentiate the effects of sparsomycin, which promotes increased aminoalcyl-tRNA binding at the P-site, while antagonizing the effects anisomycin, a drug that promotes decreased peptidyl-tRNA binding at the A-site. The changes in ribosome affinities for tRNAs also correlate with decreased peptidyltransferase activities of mutant ribosomes, and with decreased rates of cell growth and protein synthesis.In vivodimethylsulfate (DMS) protection studies reveal that small changes in L3 primary sequence also have significant effects on rRNA structure as far away as 100?, supporting an allosteric model of ribosome function.