| dc.description | Five nuclear protein-coding genes and the corresponding cDNAs have been cloned or PCR-amplified from the red alga Gracilaria verrucosa and characterized by sequencing and genomic Southern hybridization: cytosolic glyceraldehyde-3-phosphate dehydrogenase (GapC), plastid-localized GAPDH (GapA), triosephosphate isomerase (TPI1), mitochondrial aconitase (m-ACN) and polyubiquitin (UBI6R). All are single-copy. Three spliceosomal introns occur (one each near the 5$\sp\prime$ end of GapA, GapC and m-ACN). Codon usage in these G. verrucosa genes is biased, with third-position A's underrepresented. Except for TPI1, nucleotide compositions of the genes are balanced. Upstream promoter structure, downstream poly(A) processing signals, plastid and mitochondrial transit-peptide profiles, and intron splice junctions were analyzed; the former two share more in common with those of green plants than with those of animals. Phylogenetic analyses were performed on GAPDH and TPI sequences. The position of Gracilaria TPI was unstable in TPI trees, and bootstrapping revealed that the TPI sequences lack sufficient phylogenetic information. In the GAPDH tree, the red algal nuclear lineage occurred as one of the eukaryote crown taxa. However, the divergence order among these lineages is unresolvable. In the other part of the tree, red algal GapA and green plant GapA/GapB were robustly separated, and the cyanobacterial gap2 appeared consistently as a sister group to the plastid lineage. A monophyletic origin of red algal and green plant GapA/(B) from a gap of a cyanobacterium-like endosymbiont is the most straightforward interpretation, and this gene was subsequently duplicated in green plants after the divergence of red algae. There is circumstantial (but not definitive) evidence from GapA/(B) and GapC sequences and the position of introns in the plastid transit-peptide-encoding regions, that both red algae and green plants descend from a single ancestral photosynthetic eukaryote. | en_US |
