ARCHAEAL TRANSLATION INITIATION REVISITED - THE INITIATION-FACTOR-2 AND EUKARYOTIC INITIATION-FACTOR 2B ALPHA-BETA-DELTA SUBUNIT FAMILIES

As the amount of available sequence data increases, it becomes apparen t that our understanding of translation initiation is far from compreh ensive and that prior conclusions concerning the origin of the process are wrong. Contrary to earlier conclusions, key elements of translati on initiation originated at the Universal Ancestor stage, for homologo us counterparts exist in all three primary taxa. Herein, me explore th e evolutionary relationships among the components of bacterial initiat ion factor 2 (IF-2) and eukaryotic IF-2 (eIF-2)/eIF-2B, i.e., the init iation factors involved in introducing the initiator tRNA into the tra nslation mechanism and performing the first step in the peptide chain elongation cycle, All Archaea appear to posses a fully functional eIF- 2 molecule, hut they lack the associated GTP recycling function, eIF-2 B (a five-subunit molecule), Yet, the Archaea do posses members of the gene family defined by the (related) eIF-2B subunits alpha, beta, and delta, although these are not specifically related to any of the thre e eukaryotic subunits, Additional members of this family also occur in some (but by no means all) Bacteria and even in sonic eukaryotes. The functional significance of the other members of this family is unclea r and requires experimental resolution, Similarly, tile occurrence of bacterial IF-2-like molecules in all Archaea and in some eukaryotes fu rther complicates the picture of translation initiation, Overall, thes e data lend further support to the suggestion that the rudiments of tr anslation initiation were present at the Universal Ancestor stage.