Impact of a plastid-bearing endocytobiont on apicomplexan genomes

Both the chromosomal and extrachromosomal components of the apicomplexan ge nome have been supplemented by genes from a plastid-bearing endocytobiont: probably an algal cell. The sequence of the apicomplexan plastid's vestigia l genome indicates that a large number (> 100) of genes of endocytobiotic o rigin must have transferred laterally to the host cell nucleus where they c ontrol maintenance of the plastid organelle and supply its functional compo nents by means of post-translational protein trafficking. Should the nuclea r genes prove to be less divergent phylogenetically than those left on the plastid genome they might give better clues than we have at present to the origin of the plastid-bearing endocytobiont. Most of these nuclear genes st ill await discovery, but the on-going genome sequencing project will reveal the function of the organelle, as well as many "housckeeping" processes of interest on a wider front. The plastid's own protein synthetic machinery, being cyanobacterial in origin, offers conventional targets for antibiotic intervention, and this is discussed here using a structural model of elonga tion factor Tu. Uncovering the vital function(s) of the plastid organelle w ill provide new drug targets. (C) 2000 Australian Society of Parasitology I nc. Published by Elsevier Science Ltd. All rights reserved.