Presence of a chloroplast DNA sequence in an autonomous circular DNA molecule in cultured rice cells (Oryza sativa L).

Sequence analysis of twelve DNA fragments, which had previously been found to be extensively amplified in suspension-cultured rice cells, revealed that two of them, isolated on plasmids designated pE10 and pE11, have sequences identical to distinct regions of chloroplast DNA (ct-DNA). Both sequences are part of an extrachromosomal circular DNA molecule (ECD). The molecular structure of the ECD was investigated by a combination of restriction analysis, standard and pulsed-field gel electrophoresis, hybridization with ct-DNA probes and amplification by the polymerase chain reaction in the presence of oligonucleotide primers homologous to selected regions of rice ct-DNA. The results showed that a continuous and unrearranged stretch of ct-DNA from the long single-copy region, of at least 28 kbp in length, is present in the ECD. It was estimated that the number of copies of the ECD in cultured cells was almost equivalent to that of ct-DNA molecules in rice leaves, while the ratio of ECD to ct-DNA molecules in the cultured cells was approximately 200:1.