Chloroplast protein and centrosomal genes, a tRNA intron, and odd telomeres in an unusually compact eukaryotic qenome the cryptomonad nucleomorph

摘要

Cells of several major algal groups are evolutionary chimeras of two radically different eukaryotic cells. Most of these "cells within cells" lost the nucleus of the former algal endosymbiont. But affer hundreds of millions of years cryptomonads still retain the nucleus of their former red algal endosymbiont as a tiny relict organelle, the nucleo- morph. which has three minute linear chromosomes. but their func- tion and the nature of their ends have been unclear. We report extensive cryptomonad nucleomorph sequences (68.5 kb), from one end of each of the three chromosomes of Guillardia theta. Telomeres of the nucleomorph chromosomes differ dramatically from those of other eukaryotes, being repeats of the 23-mer sequence (AG)_7 AAG_6A, not a typical hexamer (commonly TTAGGG). The subterminal regions comprising the rRNA cistrons and one protein-coding gene are exactly repeated at all three chromosome ends. Gene density (one per 0.8 kb) is the highest for any cellular genome. None of the 38 protein-coding genes has spliceosomal introns, in marked contrast to the chlorarach- niophyte nucleomorph. Most identified nucleomorph genes are for gene expression or protein degradation: histone, tubulin. and puta- tively centrosomal ranbpm genes are probably important for chro- mosome segregation. No genes for primary or secondary metabolism have been found. Two of the three tRNA genes have introns, one in a hitherto undescribed location. lntergenic regions are exceptionally short: three genes transcribed by two different RNA polymerases overlap their neighbors. The reported sequences encode two essen- tial chloroplast proteins. FtsZ and rubredoxin, thus explaining why cryptomonad nucleomorphs persist.