The visualization of a single salient loop of L11 interacting with peptidyl-tRNA indicated that it might play a vital role in sensing peptidyl-tRNA occupancy status and transmitting this information to other functional centers of the ribosome. As cells expressing RPL11B alone were healthier than those solely expressing RPL11A, genetic manipulations began with the yeast rpl11AD rpl11BD double knockout strain JD1313 expressing wild-type (WT) RPL11B from a low-copy, URA3-selectable episomal plasmid (pRPL11B-URA3). Oligonucleotide site-directed mutagenesis was used to construct a series of mutants, each containing changes of 1, 4 or 10 sequential amino acids ( Figure 1C ). Stretches of amino acids from arginine 51 to arginine 61 in the L11 P-site loop were targeted for site-directed mutagenesis expressed from a low-copy, TRP1-selectable episomal plasmid under control of the endogenous RPL11B promoter (pRPL11B-TRP1). After transformation and selection on SD medium lacking tryptophan (-trp), cells expressing only mutant rpl11B alleles were identified by their ability to grow on SD-trp medium containing 5-flouroorotic acid (5-FOA). Three of the multiple substitution mutants were inviable as the sole forms of L11B. These were R 51 YTVRTFGIR 60 !alanine (i.e. 51-60A); deletion of residues 51-60 (51-60Á); and F 57 GIR 60 !alanine (57-60A). Viable mutants, R 51 YTV 54 !alanine (51-4A), V 54 RTF 57 !alanine (54-7A), R51A, Y52* (mutations including Á, A, R, E, S, I, Q, N, H and F), F57A and R61A were rescued from yeast into E. coli, and the mutations were confirmed by DNA sequencing.