Recombination within the inverted repeat sequences of the Chlamydomonas reinhardii chloroplast genome produces two orientation isomers

Summary Two orientations of the Chlamydomonas reinhardii chloroplast (ct) genome are shown to be produced by recombination within the inverted repeat (IR) sequences that separate the two single copy (SC) regions. SC region 1 is bounded on its two ends by EeoRI restriction endonuclease fragments of 3.2 and 4.7 kilobase pairs (kb) (Rochaix 1978). The 3.2 kb EeoRI fragment overlaps a 51.3 kb BglII fragment spanning one of the 19.7 kb IR sequences, and the 4.7 kb EcoRI fragment overlaps a 42.1 kb BglII fragment spanning the other 19.7 kb IR sequence. We have shown by hybridization analysis that the 3.2 kb fragment also overlaps a BgIII fragment with a predicted size of 52.3 kb, and that the 4.7 kb fragment also overlaps a BglII fragment of a predicted size of 41.1 kb. The second set of BglII fragments are isomers produced by recombination localized to the IR region. The two isomers are present in approximately equimolar ratio. Knowledge of the isomeric composition of the C. reinhardii ctDNA is essential for establishing a correlation between genetic and physical maps of the ct genome.