STRUCTURE AND COTRANSCRIPTION OF THE PLASTID-ENCODED rbcL. AND rbcS GENES OF PLEUROCHRYSIS CARTERAE (PRYMNESIOPHYTA)

Genes for the large (rbcL) and small (rbcS) subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from the prymnesiophyte Pleurochrysis carterae (Braarud and Fagerl.) Christensen were isolated and characterized. The genes are located close to each other, separated by an intergenic region of 87 nucleotide base pairs. Southern and Northern blot analyses indicate that these genes are encoded by plastid DNA and cotranscribed, as in the Cryptophyta, Chromophyta, and Rhodophyta studied so far. The genes of Pleurochrysis show higher identities with those of the Cryptophyta, Chromophyta, Rhodophyta, and the alpha (Type I)- and beta -purple bacteria than with those of the Chlorophyta or Cyanophyta. The molecular phylogenetic tree of rbcL genes suggests that the plastids of the Prymnesiophyta, Cryptophyta, and Chromophyta originated from those of the Rhodophyta, which agrees with the secondary endosymbiotic theory. Northern analysis of Pleurochrysis demonstrated that a certain amount of the transcript is possibly processed around the 3' end of rbcL. When cells were transferred from light to dark for 6 h, the amount of Rubisco messenger RNA (mRNA) was almost the same as in continuous light. This suggests that Rubisco mRNA is synthesized and/or stable even in the dark.