Plastid genome size and heterogeneous base composition of nuclear DNA fromOchrosphaera neapolitana (Prymnesiophyta)

We have studied the plastid genome of Ochrosphaera neapolitana (Hymenomonad aceae, Prymnesiophyta), a member of the chlorophyll a + c-containing algae. Total DNA from ). neapolitana was fractionated in a CsCl density gradient, and the 'lighter' upper band (which is typically of plastid origin) was un usually abundant relative to the 'heavier' nuclear one. Denaturation and re naturation experiments done with DNA isolated from the upper band showed th at it had a base composition of 43.6 mol% G + C (molar fraction of guanosin e plus cytosine), compared with 60.5 mol% G + C in the lower band, and that it lacked a substantial fraction of repetitive sequences; this DNA had a s ize of c. 10 Mb, which is more than an order of magnitude larger than has b een reported for any plastid genome. Using pulse field gel electrophoresis (PFGE) of total DNA, we resolved a plastid genome size of approximately 165 kb for O. neapolitana. Although this is within the range reported for othe r plastid genomes, there is an increase of about 40 kb compared with relate d algae. Using the upper band from the CsCl gradient as a probe for the PFG E blots, we showed that this upper band is apparently a mixture of nuclear DNA with a small fraction of plastid DNA. The nuclear genome of O. neapolit ana thus appears to have a distinctive heterogeneity in G + C content, with two DNA types differing by c. 17 mol%.