Exploring the limits of codon and anticodon size.

We previously employed a combinatorial approach to identify the most efficient suppressors of four-base codons in E. coli. We have now examined the suppression of two-, three-, four-, five-, and six-base codons with tRNAs containing 6-10 nt in their anticodon loops. We found that the E. coli translational machinery tolerates codons of 3-5 bases and that tRNAs with 6-10 nt anticodon loops can suppress these codons. However, N-length codons were found to prefer N + 4-length anticodon loops. Additionally, sequence preferences, including the requirement of Watson-Crick complementarity to the codon, were evident in the loops. These selections have yielded efficient suppressors of four-base and five-base codons for our ongoing efforts to expand the genetic code. They also highlight some of the parameters that underlie the fidelity of frame maintenance.