GLOBINS IN NONVERTEBRATE SPECIES - DISPERSAL BY HORIZONTAL GENE-TRANSFER AND EVOLUTION OF THE STRUCTURE-FUNCTION-RELATIONSHIPS

Using a new template based on an alignment of 145 nonvertebrate globin s we examined several recently determined sequences of putative globin s and globin-like hemeproteins. We propose that all globins have evolv ed from a family of ancestral, approx. 17-kDa hemeproteins, which disp layed the globin fold and functioned as redox proteins. Once atmospher ic O-2 became available the acquisition of oxygen-binding properties w as initiated, culminating in the various highly specialized functions known at present. During this evolutionary process, we suggest that (1 ) high oxygen affinity may have been acquired repeatedly and (2) the f ormation of chimeric proteins containing both a globin and a flavin bi nding domain was an additional and distinct evolutionary trend. Furthe rmore, globin-like hemeproteins encompass hemeproteins produced throug h convergent evolution from nonglobin ancestral proteins to carry out O-2-binding functions as well as hemeproteins whose sequences exhibit the loss of some or all of the structural determinants of the globin f old. We also propose that there occurred two cases of horizontal globi n gene transfer, one from an ancestor common to the ciliates Parameciu m and Tetrahymena and the green alga Chlamydomonas to a cyanobacterium ancestor and the other, from a eukaryote ancestor of the yeasts Sacch aromyces and Candida to a bacterial ancestor of the proteobacterial ge nera Escherichia, Alcaligenes, and Vitreoscilla.