Friday, January 26, 2007

Not so silent mutations

DNA mutations that do not change the coding amino-acid are many times referred to as "silent mutations", or synonymous mutations, because it is less likely that they will result in a change in function. Synonymous mutations are often considered to be evolutionary neutral and the ratio of non-synonymous substitutions (Ka) to synonymous substitutions (Ks) is used to study sequence evolution. It can be used for example to search for DNA regions targeted by selection (see review and a practical application).

In the last issue of Science Kimchi-Sarfaty and colleagues found a synonymous mutation in a transport protein that has an effect on the protein function. They have shown, at least in cell-lines, that the mutation does not affect mRNA levels nor the produced protein sequence. Finally the authors showed that the mutation might change the protein's conformation by comparing the sensibility of wild type and mutated sequence to trypsin digestion.

The authors speculate that the usage of that particular codon, even if not affecting the coding region, might change the translation rate and folding of the protein. It had already been shown in E. coli that synonymous mutations can affect the in vivo folding of a protein. Here the authors have shown a case where a silent mutation can change the substrate specificity of a transporter.

Because of these codon preferences it is important to adjust for codon selection pressures when studying synonymous substitutions. The codon preferences are usually considered to be due to differences in the pool of the cognate tRNA but other studies have shown that codon bias might arise also by codon context. In E. coli, codon pair preferences, were observed to affect their in vivo translation. Also, these codon pair preferences are species specific and are, at least in part, influenced by nucleotide positions within A-site tRNA sequences.

Hypothesis: If codon pairs can be selected due to tRNA structural constrains on the ribosome P and A sites then it might be necessary to correct for these codon preferences when studying synonymous mutations.