By overexpression of modified Escherichia coli 23S rRNAs from multicopy plasmids, ribosomes were prepared that contained mutations in two regions (2447-2450 and 2457-2462) of 23S rRNA. Following mutagenesis and selection, two clones with mutations in the 2447-2450 region (peptidyltransferase center) and six with mutations in the 2457-2462 region (helix 89) were characterized. The mutations were shown to exhibit a high level of homology. Cell-free protein synthesizing systems prepared from these clones were found to exhibit significantly enhanced incorporation of d-methionine and d-phenylalanine into protein. The incorporations involved positions 10, 22, and 54 of E. coli dihydrofolate reductase and positions 247 and 250 of Photinus pyralis firefly luciferase. Interestingly, some of the derived proteins containing the d-amino acids (notably DHFR analogues altered at position 10) functioned as well as those containing the respective l-amino acids, while substitution at other positions resulted in proteins having greatly diminished activity.

While numerous biologically active peptides contain D-amino acids, the elaboration of such species is not carried out by ribosomal synthesis. In fact, the bacterial ribosome discriminates strongly against the incorporation of D-amino acids from D-aminoacyl-tRNAs. To permit the incorporation of D-amino acids into proteins using in vitro protein-synthesizing systems, a strategy has been developed to prepare modified ribosomes containing alterations within the peptidyltransferase center and helix 89 of 23S rRNA. S-30 preparations derived from colonies shown to contain ribosomes with altered 23S rRNAs were found to exhibit enhanced tolerance for D-amino acids and to permit the elaboration of proteins containing D-amino acids at predetermined sites. Five specific amino acids in Escherichia coli dihydrofolate reductase and Photinus pyralis luciferase were replaced with D-phenylalanine and D-methionine, and the specific activities of the resulting enzymes were determined.

Ribosomally mediated protein biosynthesis is limited to α-L-amino acids. A strong bias against β-L-amino acids precludes their incorporation into proteins in vivo and also in vitro in the presence of misacylated β-aminoacyl-tRNAs. Nonetheless, earlier studies provide some evidence that analogues of aminoacyl-tRNAs bearing β-amino acids can be accommodated in the ribosomal A-site. Both functional and X-ray crystallographic data make it clear that the exclusion of β-L-amino acids as participants in protein synthesis is a consequence of the architecture of the ribosomal peptidyltransferase center (PTC). To enable the reorganization of ribosomal PTC architecture through mutagenesis of 23S rRNA, a library of modified ribosomes having modifications in two regions of the 23S rRNA (2057-2063 and 2496-2507 or 2582-2588) was prepared. A dual selection procedure was used to obtain a set of modified ribosomes able to carry out protein synthesis in the presence β-L-amino acids and to provide evidence for the utilization of such amino acids, in addition to α-L-amino acids. β-Puromycin, a putative mimetic for β-aminoacyl-tRNAs, was used to select modified ribosome variants having altered PTC architectures, thus potentially enabling incorporation of β-L-amino acids. Eight types of modified ribosomes altered within the PTC have been selected by monitoring improved sensitivity to β-puromycin in vivo. Two of the modified ribosomes, having 2057AGCGUGA2063 and 2502UGGCAG2507 or 2502AGCCAG2507, were able to suppress UAG codons in E. coli dihydrofolate reductase (DHFR) and scorpion Opisthorcanthus madagascariensis peptide IsCT mRNAs in the presence of β-alanyl-tRNA(CUA).

Many pathogens utilize the formation of trans-membrane pores in target cells in the process of infection. A great number of pore-forming proteins, both bacterial and viral, are considered to be important virulence factors, which makes them attractive targets for the discovery of new therapeutic agents. Our research is based on the idea that compounds designed to block the pores can inhibit the action of virulence factors, and that the chances to find high affinity blocking agents increases if they have the same symmetry as the target pore. Recently, we demonstrated that derivatives of β-cyclodextrin inhibited anthrax lethal toxin (LeTx) action by blocking the trans-membrane pore formed by the protective antigen (PA) subunit of the toxin. To test the broader applicability of this approach, we sought β-cyclodextrin derivatives capable of inhibiting the activity of Staphylococcas aureus α-hemolysin (α-HL), which is regarded as a major virulence factor playing an important role in staphylococcal infection. We identified several amino acid derivatives of β-cyclodextrin that inhibited the activity of α-HL and LeTx in cell-based assays at low micromolar concentrations. One of the compounds was tested for the ability to block ion conductance through the pores formed by α-HL and PA in artificial lipid membranes. We anticipate that this approach can serve as the basis for a structure-directed drug discovery program to find new and effective therapeutics against various pathogens that utilize pore-forming proteins as virulence factors.