Novel plastid gene minicircles in the dinoflagellate Amphidinium operculatum

Seven new minicircles, forming part of the fragmented plastid genome of the dinoflagellate Amphidinium operculatum, have been identified by PCR. Three minicircles are full-length, one of 2.6 kb encoding the 23S rRNA gene, one of 2.4 kb containing the psaB gene, and a third of 2.5 kb containing the psbD, psbE and psbI genes. This is the first report of a three-gene minicircle. All three genes are conventional in length, with the same codon bias found in other minicircle genes. The psbI gene sequence is very divergent. One empty minicircle, of 1.7 kb, and three 'microcircles', between 400 and 600 bp, have also been identified by PCR. They appear to be formed from full-length minicircles by homologous recombination and internal deletion. All three microcircles contain the core region common to minicircles, but are missing a coding region, providing further evidence that the core region is necessary for minicircle replication and maintenance.