Chloroplasts, bacterial origin

Plastocyanins are the most widely studied cupredoxins. They are one of the most abundant copper proteins in plant photosynthetic tissues. Plant plastocyanins have an intricate evolutionary history because of their ancient bacterial origin. It is currently well accepted that plants diverged from the main eukaryotic domain into a separate lineage when the unicellular, oxygen respiring common ancestor of the eukaryotes incorporated a prokaryotic endosymbiont, the cyanobacterial chloroplast. 

This would favour hypothesis II in Fig. 7.15. Even animals share some of the proteins (TDC, ODC, TyrDC, STS and CR). These observations indicate that the proteins very likely evolved in prokaryotes and were transferred into eucaryotes via either protobacteria (Fig. 7.18) or cyanobacteria, the progenitors of mitochondria or chloroplasts, respectively. A number of SM (e.g. many terpenoids, QAs, the piperidine alkaloid coniine) are produced completely or partly in chloroplasts and/or mitochondria (see Chapter 1). The corresponding genes are mostly nuclear today. It is tempting to speculate that these localizations are indirect indicators of a former bacterial origin of the corresponding pathways. The introduction of bacterial genomes into eukaryotes was... 

The chloroplast genome is similar to that of mitochondria, reflecting its similar bacterial origin. However, the circular chromosome is larger than its mitochondrial counterpart, encoding 30 membrane proteins that are involved in photosynthesis. It also encodes the four components of the bacterial ribosomal system (23S, 16S, 5S, and 4.5S), 20 ribosomal proteins, and 30 tRNAs. Notable among the proteins is one of the subunits of ribulose bisphosphate carboxylase (Rubisco). This enzyme catalyzes the carboxylation of ribulose 1,5-bisphosphate from COj and is responsible for carbon fixation in plants. It is the most abimdant protein on earth. 

At first, trees depicting a symbiotic origin of mitochondria and chloroplasts were sketchy, and were cobbled together from partial sequences of diverse molecules, 5S RNA, ferredoxin, cytochrome c, based on very few organisms (Schwartz and Dayhoff 1978). They were easily criticized (Demoulin 1979). The decisive evidence came from systematic investigations of bacterial phylogeny based on comparisons of the small subunit ribosomal RNA (SSU rRNA) pioneered by the group led by Carl Woese at the University of Illinois. 

Whatever the explanation of the stem eukaryote, the eukaryote organelles, both mitochondria and chloroplasts, are best explained as symbiont bacteria. Explanations of the mitochondrial symbiosis mostly invoke an early Archean stem that incorporated a bacterial symbiont. One explanation of the mitochondrion is that the origin of the mitochondrion was simultaneous with the origin of the eukaryote nucleus (Grey et al, 1999). In the hydrogen hypothesis (Martin and Muller, 1998), the symbiosis is seen as the end product of a tight physical association between anaerobic... 

Chromatophores 1. plastids of higher plants Chloroplasts (see), Chromoplasts (see) and Leuco-plasts (see). 2. The photosynthetic organelle of Photosynthetic bacteria (see). Bacterial C. are intraplasmatic membranes originating from the cell membrane. They may exist as closed vesicles or as flattened stacks, whose membranes contain the photosynthetic pigments, and the components of photosynthetic electron transport and photophosphorylation. 

---