The late stage of genetic code structuring took place at a high temperature

The correlation between the optimal growth temperature of organisms and a t hermophily index based on the propensity of amino acids to enter more frequ ently into (hyper)thermophile proteins is used to conduct an analysis aimin g to establish whether genetic code structuring took place at a low or a hi gh temperature. If the number of codons attributed to the various amino aci ds in the genetic code constitutes an estimate of the mean amino acid compo sition of proteins produced when the genetic code was definitively structur ed, then the thermophily index can also be associated to the genetic code. This value and the sampling of the variable thermophily index of different alignments of protein sequences from mesophile, thermophile and hyperthermo phile species make it possible to establish, with an extremely high statist ical confidence, that the late stage of genetic code structuring took place in a hyperthermophile (or thermophile) 'organism'. Moreover the 95% confid ence interval of the temperature at which the genetic code was fixed turned out to be 91 +/- 24 degreesC. These observations seem to support the hypot hesis that the origin of life might have taken place at a high temperature. (C) 2000 Elsevier Science B.V. All rights reserved.