ABSTRACT
Rccent crystallographic and kinetic data have provided a detailed description o f decoding in
protein synthesis. Selection o f a tRNA can roughly be divided into two stages. During initial selec tion, the ribosome determines if a given mRNA-tRNA pairing is cognate based on the formation o f Watson-Crick base pairs in the first two positions o f the codon-anticodon minihelix. At this point noncognate tRNAs are completely rejected, while near-cognate tRNAs are strongly discriminated against with higher off rates and lower rates of GTPase activation compared to cognate tRNAs (ap proximately 10-fold). This results in at least a 10 fold increase o f the GTP hydrolysis rate o f EF-Tu for cognate compared to near-cognate tRNAs. The GTP hydrolysis rate for noncognate tRNAs is about a 100,000 fold lower. Following GTP hydrolysis by EF-Tu, a second selection step takes place, for which the rate for tRNA dissociation is about a 100 fold higher for near-cognate tRNAs compared to cognate (for a more detailed discussion see ref. 5). During this second selection step, the overall energy o f binding o f the tRNA to the codon is a critical component.
