The relationship of a prochlorophyte Prochlorothrix hollandicato green chloroplasts

Abstract

It is generally accepted that chloroplasts arose from one or more endosymbiotic events between an ancestral cyanobacterium and a eukaryote. Such an origin fits well in the case of the chloroplasts of rhodophytes that, like cyanobacteria, contain chlorophyll a and phycobilin pigments. The green chloroplasts from higher plants, green algae, and euglenoids however, contain chlorophyll b as well as chlorophyll a, and lack phycobilins. Consequently, it has been suggested that they arose independently of the rhodophyte chloroplasts, from an ancestral prokaryote containing that complement of pigments. The 'prochlorophytes' Prochloron didemni (an exosymbiont on didemnid ascidians) and Prochlorothrix hollandica (a recently discovered, free-living, filamentous form) have been suggested to be modern counterparts of the ancestor of the green chloroplasts because they are prokaryotes that also contain both chlorophylls a and b, and lack phycobilins. We report here a 16S rRNA-based phylogenetic analysis of P. hollandica. The organism is found to fall within the cyanobacterial line of descent, as do the green chloroplasts, but it is not a specific relative of green chloroplasts. Thus, similar pigment compositions do not necessarily reflect close evolutionary relationships.

The cyanelle genome of Cyanophora paradoxa encodes ribosomal proteins not encoded by the chloroplast genomes of higher plants

“…Modern procaryotic and eucaryotic algal descendents would have retained one or the other antennae system, but no cyanobacteria have yet been described which exhibit both potentials. This scenario nicely explains why all oxygen-evolving procaryotes and plastids form a single cluster when analyzed on the basis of 16s rRNA similarities [18,47].…”

“…In contrast, the gene order 5 '-rpl5-rpsl4-rps8-rp16-3 ' has been shown to be conserved in diverse procaryotes including M. There are essentially two ways to view the cyanelle: (1) as a relatively recent reinvention of the type of endosymbiotic event which led to chloroplasts and in which the cyanelle is convergently evolving towards the chloroplast; or (2) as a distinctive modern-day descendent from an ancestral organism that possibly evolved into organisms harboring chloroplasts on the one hand and cyanelles on the other. Although there are conflicting views concerning the origins of higher plant chloroplasts [17,47,48], recent evidence suggests that view 2 may be correct and that the phycobiliprotein-containing cyanelles are in fact more closely related to the chloroplasts of higher plants than are the chloroplasts of E. gracilis and C~lamydomonas rein~ardtii [ 17,421. This presents an apparent paradox, since the green algae share with the higher plants and certain procaryotes such as Prochloron sp.…”

Localization and expression of the closely linked cyanelle genes for RNase P RNA and two transfer RNAs

“…The petB-petD operons have retained their position relative to IR regions in all chloroplast genomes; psbA, which is close to trnH in chloroplasts, is located ahnost 50 kbp away on the LSC region in cyanelles. Thus, we must assume several major recombination events after the divergence [7] of ancestral cyanelles and chloroplasts, which finally led to the apparent loss in chloroplasts of a~ essential RNA gene that is well preserved in cyanelles. …”

“…One putative intermediate in plastid evolution is the unicellular photosynthetic protist Cyanophora paradoxa. The pigment and cell wall composition of its photosynthetic orgenelles or cyanelles resemble that of free-living ~,yan~bacteria, whereas the dramatically reduced size and orgstnization of their genome is more similar to chloroplasts [4,$|, Sequence derived phylogenetic trees position the cyanelle in a coherent group together with cyanobacteria and green plastids [6,7]; several genes that hstve been lost from chloropint genomes early in plant evolution are present in the cyahelle genome, Taken together, these facts support the idea that this orpnelle might be considered st remnant of an early star.ca of plastid evolution, and thus serve as a model to study the molecular evolution of RNstse P, an ancient and ubiquitous RNA enzyme, in plastids of different origin.…”

The Molecular Evolution and DNA Profiling of Toxic Cyanobacteria

“…with strains of Synechocystis. However, studies on the molecular diversity of this group of bacteria, which contain both chlorophyll a and b, has revealed it to be polyphyletic with comparatively large genetic distances between representative strains (12)(13)(14).…”