Unusual characteristics of Codium fragile chloroplast DNA revealed by physical and gene mapping.

A complete physical map of the Codium fragile chloroplast genome was constructed and the locations of a number of chloroplast genes were determined. Several features of this circular genome are unusual. At 89 kb in size, it is the smallest chloroplast genome known. Unlike most chloroplast genomes it lacks any large repeat elements. The 8 kb spacer region between the 16S and 23S rRNA genes is the largest such spacer characterized to date in chloroplast DNA. This spacer region is also unusual in that it contains the rps12 gene or at least a portion thereof. Three regions polymorphic for size are present in the Codium chloroplast genome. The psbA and psbC genes map closely to one of these regions, another region is in the spacer between the 16S and 23S rRNA genes and the third is very close to or possibly within the 16S rRNA gene. The gene order in the Codium genome bears no marked resemblance to either the "consensus" vascular plant order or to that of any green algal or bryophyte genome.