論文

Cyanidioschyzon merolae is a small unicellular red alga that is considered to belong to one of the most deeply branched taxa in the plant kingdom. Its genome size is estimated to be 16.5 Mbp, one of the smallest among free-living eukaryotes. In the nucleus containing this small genome, one nucleolus is clearly observed, but the molecular basis for the intranuclear structure including ribosomal DNA organization is still unclear. We constructed a bacterial artificial chromosome library for C. merolae 10D composed of two subsets with different insert size distributions. The two subsets have average insert sizes of 97 and 48 kb, representing 10.0- and 6.9-fold genome-equivalent coverage of the haploid genome, respectively. For application to whole-genome shotgun sequencing, the termini of each clone were sequenced as sequence-tagged connectors and mapped on the contigs assigned to chromosomes. Screening for rRNA genes by conventional colony hybridization with high-density filter blots and subsequent sequencing revealed that the C. merolae genome contained the smallest number of ribosomal DNA units among all the eukaryotes examined to date. They consist of only 3 single units of rRNA genes distributed on separate chromosomal loci, representing an implication for concerted evolution. Based on these results, the origin and evolution of the nucleolus are discussed.

We have previously identified a cluster of 16 genes that encode hair-specific proteins, called keratin-associated proteins (KAPs), located on human Chromosome (Chr) 21q22.3. Here, we have identified similar KAP gene clusters in two primates, chimpanzee and baboon. DNA sequence comparison revealed the common cluster structure consisting of 16 KAP genes for these three primates, but a significant difference was found in the baboon. Baboon possesses a new KAP gene not found in human and chimpanzee, whereas one KAP gene ( KRTAP18.12) that exists in human and chimpanzee was lost in baboon, making no change in the total number of KAP genes. Interestingly, the sequence for coding regions are highly variable among species owing to insertions and deletions, resulting in variation of gene size. On the contrary, the sequences for the 5' upstream region are highly conserved among species. These findings suggest that the ancestral KAP gene cluster was composed of 17 genes before the divergence of Old World monkeys (baboon) to the anthropoid (human and chimpanzee).