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As an initial step for understanding plastid-specific gene regulation during plastid transformations and phylogenetic studies within Cucurbita species, plastid DNAs were isolated from the chloroplasts and chromoplasts of squash (Cucurbita pepo L.). Crude chloroplasts and chromoplasts from leaves and yellow fruits were purified by centrifugation through continuous (30-60%) and discontinuous (30/40/60%) sucrose density gradients, respectively. The isolation methods of squash chromoplasts were quite different from those of chromoplasts of red peppers and tomatoes. High quality chloroplast and chromoplast DNAs without obvious contamination by either mitochondrial or nuclear DNAs were obtained by extensive phenol/chloroform extractions. The isolation methods of plastid DNAs developed in this study were rapid and simple, and required no CsCl density centrifugation. Several alternative procedures for plastid DNA isolation were also briefly discussed. The restriction fragment patterns of chloroplast and chromoplast DNAs produced by digestions with various restriction endonucleases (SalI, PvuII, SalI+PvuII, Pst I, Hap II, Bgl I, Sac II, and Bst NI) were compared, and they appeared to be identical between chloroplast and chromoplast DNAs. These results suggest that chloroplasts and chromoplasts contain an identical genome.
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