Optimum growth temperature and the base composition of open reading frames in prokaryotes.

The purine-loading index (PLI) is the difference between the numbers of purines (A+G) and pyrimidines (T+C) per kilobase of single-stranded nucleic acid. By purine-loading their mRNAs organisms may minimize unnecessary RNA-RNA interactions and prevent inadvertent formation of "self" double-stranded RNA. Since RNA-RNA interactions have a strong entropy-driven component, this need to minimize should increase as temperature increases. Consistent with this, we report for 550 prokaryotic species that optimum growth temperature is related to the average PLI of open reading frames. With increasing temperature prokaryotes tend to acquire base A and lose base C, while keeping bases T and G relatively constant. Accordingly, while the PLI increases, the (G+C)% decreases. The previously observed positive correlation between (G+C)% and optimum growth temperature, which applies to RNA species whose structure is of major importance for their function (ribosomal and transfer RNAs) does not apply to mRNAs, and hence is unlikely to apply generally to genomic DNA.