Phytogene Sequence: From Cyanobacteria to Plastids
Sugiura, M.
Center for Gene Research, Nagoya University, Nagoya 464-8602, Japan
The endosymbiotic theory has proposed that plastids (chloroplasts) were derived from an ancestral photosynthetic prokaryote related to cyanobacteria. This idea was supported at the molecular level by the observations that nucleotide sequences of plastid 5S rRNAs and 16S rRNA genes are highly similar to those from the unicellular cyanobacterium Anacystis nidulans. Further analysis of plastid as well as cyanobacterial genomes provides the fundamental data needed to estimate the origin and evolution of plastids.
As plastid DNA molecules are y small and simple, they were selected as one of the first targets of “the genome projects”. The entire nucleotide sequences of tobacco and 14 other plastid genomes have been determined to date. For example, the tobacco plastid genome is 156 kbp in length and contains four different rRNA genes, 31 different tRNA genes and at least 70 different protein-coding genes. However, recent analysis of plastid DNAs from algae, especially red and brown algae, has revealed the presence of many novel genes not found in land plant plastids.
A complete coding information was initially thought to be written in a DNA sequence. However this is not always the case at least in some plastids, and DNA information is often modified at the level of RNA, by RNA editing as well as RNA splicing. Therefore, plastid protein structures or protein-coding regions cannot always be predicted from their genome sequences.
Physical and gene maps of several representative cyanobacterial genomes have been reported. Their genome sizes are from 2.7 Mb (Synechococcus 6301 and 7002) to 6.4 Mb (Anabaena 7120). Among them, Synechocystis 6803 was chosen to determine the entire genome sequence bercause of its moderate GC content and a popular strain. The genome size determined is 3,573,470 bp with the GC content of 47.8 %. It was found to contains two rRNA gene clusters, 42 tRNA genes encoding 41 tRNA species, 3 small stable RNA genes, and 3,168 ORFs, among which 55% show similarity to known and hypothetical genes.
| LOCATION |
DATE |
TIME |
| Lecture Hall I |
Tuesday, April 7 |
06:20 pm |