Encoding of both subunits of ribulose-1,5-bisphosphate carboxylase by organelle genome of Cyanaphora paradoxa

Abstract

The intracellular photosynthetic entities (cyanelles) of the protozoan Cyanophora paradoxa (Fig. 1) have long been regarded as endosymbiotic cyanobacteria due to morphological and physiological similarities such as unstacked thylakoids¹, a rudimentary yet lysozyme-sensitive cell wall^2,3, and phycobilisomes as light-collecting apparatus⁴. However, recent investigations of cyanelle DNA cast doubt on this classification. The cyanelle genome has been found to resemble chloroplast DNA in size and structure^5–7. In light of the endosymbiont hypothesis of eukaryotic cell evolution⁸ these paradoxical features of the cyanelles suggest that C. paradoxa is an evolutionary intermediate between cyanobacteria, the presumed plastidal ancestors, and modern photosynthetic organelles. The ribulose-1,5-bisphosphate carboxylase (RuBPCase) of C. paradoxa is composed of eight large (LSU) and eight small (SSU) subunits⁹. In higher plants and green algae the LSU of the enzyme is encoded by plastid DNA and the SSU is nucleus-encoded¹⁰. In cyanobacteria the LSU gene has been located on the chromosome¹¹. No data are available on the SSU gene. Here we have investigated the cyanelles' evolutionary position by determining the location of the genes for the two subunits of RuBPcase in C. paradoxa. By heterologous hybridization the genes for both subunits of RuBPCase have been located on the cyanelle genome. Furthermore, LSU and SSU probes exhibit homology to the same BglII and HindIII fragment, suggesting a close proximity of the two genes.