Evolution of the chloroplast genome.

We discuss the suggestion that differences in the nucleotide composition between plastid and nuclear genomes may provide a selective advantage in the transposition of genes from plastid to nucleus. We show that in the adenine, thymine (AT)-rich genome of Borrelia burgdorferi several genes have an AT-content lower than the average for the genome as a whole. However, genes whose plant homologues have moved from plastid to nucleus are no less AT-rich than genes whose plant homologues have remained in the plastid, indicating that both classes of gene are able to support a high AT-content. We describe the anomalous organization of dinoflagellate plastid genes. These are located on small circles of 2-3 kbp, in contrast to the usual plastid genome organization of a single large circle of 100-200 kbp. Most circles contain a single gene. Some circles contain two genes and some contain none. Dinoflagellate plastids have retained far fewer genes than other plastids. We discuss a similarity between the dinoflagellate minicircles and the bacterial integron system.