Evolution of E. coli tRNA(Ile): evidence of derivation from other tRNAs.

Two E. coli tRNA(Ile) sequences were compared against those of 36 other E. coli tRNAs. tRNA(Ile) 1 was found to bear high similarity with tRNA(Val) 1 (E = 1.11 X 10(-18] while tRNA(Ile) 2 had the greatest match (E = 3.40 X 10(-19] with tRNA(Lysl) (E is the expected number of such matches, per search, based on coincidence). These matches, which we consider to represent homologies, extend from base 7 to base 67 in the former and base 7 to the end (76) in the latter pair. These results coupled with others on the lower activity of isoleucine in reactions postulated to be important in primitive protein synthesis (i.e., esterification reactions and non-enzymatic activation by ATP [1-3]) lead us to propose that isoleucine was included among the proteinaceous amino acids, and received its anticodonic assignment, relatively late in evolution through mutation of tRNAs previously employed for other amino acids.