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Nucleotide Sequence of Cyanophora paradoxa Cellular and Cyanelle‐Associated 5S Ribosomal RNAs

Cyanophora poradoxa is a flagellated protozoan which has long been of particular interest because of its unique photosynthetic plastid or cyanelle. This cyanelle possesses morphological and genetic characteristics that indicate a close phylogenetic relationship with both prokaryotic cyanobacteria and eukaryotic higher plant chloroplasts!-' It has therefore been postulated that these cyanelles might represent a potential intermediate in photosynthetic plastid evolution between cyanobacteria and plant chloroplasts.' In an effort to understand this evolutionary relationship better, we have isolated and sequenced both the cellular and cyanelle-associated S S ribosomal RNAs. Cyanophora parudoxa cellular and cyanellar SS rRNAs were 119 and 118 nucleotides in length, respectively (FIGURE 1). Both exhibited the typical SS rRNA secondary structures consisting of five helical regions. Analysis of conserved nucleotides, helix and loop sizes, and defined spacings demonstrated a eukaryotic SS structure for the cellular species and a prokaryotic structure for the cyanellar species.' Sequence comparison of Cyanophoroparodoxa cellular and cyanellar SS rRNAs with other 5s rRNA sequences is shown in TABLE 1. Analysis of the cellular SS rRNA revealed a relatively low degree of homology between Cyanophoro and other eukaryotes, indicating a unique position for this organism in evolution. Highest homology was observed with Euglena gracilis SS rRNA. The cyanelle-associated SS rRNA exhibited high homology with both cyanobacterial and chloroplast SS sequences. Highest homology was observed between the cyanellar sequence and the SS rRNAs of the cyanobacterium Synechococcus lividus Interestingly, Synechococcus lividus 111 has been postulated as a possible progenitor of plant chloroplasts because of its unique chloroplastlike SS rRNA secondary structure.6 Sequence results obtained in these experiments are consistent with the idea that the cyanelle could represent an intermediate in chloroplast evolution from endocytosed cyanobacteria as postulated by the endosymbiont hypothesis.' It is impossible to say at this time whether the cyanelle is a direct precursor to chloroplasts (monophyletic event in endocytotic development of the chloroplast) or is the result of a similar but separate endocytotic event (polyphyletic) in photosynthetic plastid development.