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Mechanism of Bacterial Photosynthesis (With Diagram)

These folds contain 3 types of molecular complexes involved in photosynthesis:1) Antenna or

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Rhodospirillum, Rhodobacter) and purple sulphur bacteria

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These are the only bacteria that have oxygen-evolving photosynthesis like that of plants.

This use of ammonia in the synthesis of organic molecules can be regarded as a process for storing a valuable nutrient, but is also an important detoxifying mechanism.

Structure of leaf & green plastid , Mechanism of photosynthesis …

Contrast this with purple sulphur bacteria which are obligate phototrophs.

Purple nonsulfur bacteria grow photoheterotrophically by using light for energy and organic compounds for carbon and electrons. Disrupting the activity of the CO2-fixing Calvin cycle enzyme, ribulose 1,5-bisphosphate carboxylase (RubisCO), prevents photoheterotrophic growth unless an electron acceptor is provided or if cells can dispose of electrons as H2. Such observations led to the long-standing model wherein the Calvin cycle is necessary during photoheterotrophic growth to maintain a pool of oxidized electron carriers. This model was recently challenged with an alternative model wherein disrupting RubisCO activity prevents photoheterotrophic growth due to the accumulation of toxic ribulose-1,5-bisphosphate (RuBP) (D. Wang, Y. Zhang, E. L. Pohlmann, J. Li, and G. P. Roberts, J. Bacteriol. 193:3293-3303, 2011, ). Here, we confirm that RuBP accumulation can impede the growth of Rhodospirillum rubrum (Rs. rubrum) and Rhodopseudomonas palustris (Rp. palustris) RubisCO-deficient (ΔRubisCO) mutants under conditions where electron carrier oxidation is coupled to H2 production. However, we also demonstrate that Rs. rubrum and Rp. palustris Calvin cycle phosphoribulokinase mutants that cannot produce RuBP cannot grow photoheterotrophically on succinate unless an electron acceptor is provided or H2 production is permitted. Thus, the Calvin cycle is still needed to oxidize electron carriers even in the absence of toxic RuBP. Surprisingly, Calvin cycle mutants of Rs. rubrum, but not of Rp. palustris, grew photoheterotrophically on malate without electron acceptors or H2 production. The mechanism by which Rs. rubrum grows under these conditions remains to be elucidated.

Purple nonsulfur bacteria (PNSB) are renowned for their ability to employ versatile metabolic modules to thrive under different growth conditions. PNSB can grow photoautotrophically using light for energy, inorganic compounds other than water (e.g., thiosulfate, Fe2+) for electrons, and CO2 for carbon. The Calvin cycle is well-known for permitting autotrophic growth by converting CO2 into organic precursors for biosynthesis (). In this pathway phosphoribulokinase (PRK) expends ATP to generate ribulose 1,5-bisphosphate (RuBP). RuBP is then combined with CO2 via ribulose 1,5-bisphosphate carboxylase (RubisCO), resulting in two molecules of 3-phosphoglycerate. CO2 fixation generates relatively oxidized metabolites that accept electrons from NAD(P)H via glyceraldehyde-3-phosphate dehydrogenase.

17/02/2011 · The mechanism of photosynthesis, ..

The diffusion of H2S from the sediment into the water column enables anaerobic photosynthetic bacteria to grow.

IM carriers enter mitochondria through the OM β-barrel protein pore Tom40, helped by the OM receptor Tom70, periplasmic chaperones and the IM protein-inserter Tim22 complex (; ; ; d). Tom40 evolves sufficiently slowly for homologues to be detected in all eukaryotes but not slowly enough for its prokaryote ancestor to be unambiguously identified. I suggest that it evolved from a proteobacterial β-barrel protein like usher (), which secretes pilus proteins using periplasmic chaperones; usher and Tom40 are the only known β-barrel proteins with two pores (); a relative of both was probably already present in the proteobacterial ancestor of mitochondria (a). As soon as a new carrier evolved in the host cytosol, it could have entered the periplasm through this pore and interact with pre-existing periplasmic chaperones; in the absence of Tim22 it must either have inserted itself into the IM or, more likely, inserted with the help of pre-existing YidC (b) or another IM protein (see below). Even if import and insertion were inefficient and even if insertion were random with half the carriers entering the OM, it would supply photosynthesate to the host and initiate enslavement by providing a selective advantage for improving carrier import.

One key protein, Tom70, the receptor for importing carriers and Tim22, probably came neither from host nor symbiont, as I find its only strong Blast hits are to cyanobacteria. As suggested previously (), during the origin of mitochondria the host possibly also harboured symbiotic cyanobacteria, using its waste CO2 and providing oxygen for the mitochondrion. Such intracellular synergy could have allowed this phagotrophic/photosynthetic consortium to out-compete other protoeukaryotic competitors. However, cyanobacteria were not themselves successfully enslaved until somewhat later, by a bikont host after the primary unikont/bikont bifurcation of eukaryotes (; ). Acquisition of Tom70 from a cyanobacterial symbiont or cyanobacterial food, and its insertion by its N-terminal helical hydrophobic tail, would improve efficiency of carrier import and allow import of other IM proteins with a sufficiently similar structure that it could recognize, notably Tim22—probably initially selected to increase specificity and speed of carrier insertion into the IM (c). Tim22, strikingly, acts as receptor, pore and energy transducer for insertion—all-in-one (); adding it alone would give a specific insertion mechanism for multitopic IM proteins. With a basically efficient mechanism for importing carriers (c), extra proteins could be added to Tom40 to increase stability (Tom6, Tom7) and similarly to Tim22 (e.g. Tim54, which evolves too fast to be sure that it was present in the cenancestor and not an opisthokont invention).

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  • 2.3 Mechanism of Photosynthesis in the Purple ..

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  • purple sulphur bacteria purple non ..

    The purple bacteria are one of the groups of photosynthetic bacteria ..

  • sulfur bacteria 750 Bacteriochlorophyll Purple sulphur bacteria ..

    (each group appears to be an assembly of different lineages that evolved separately): purple non-sulphur bacteria (e.g

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On the metabolism of purple sulphur bacteria in ..

A general feature of the present scenario that space limitations did not permit me to detail is that it well fits the principle that ‘ontogeny recapitulates phylogeny’, i.e. the postulated evolutionary sequence of addition of Tom and Tim subunits generally follows that still observed today, as do their assumed import mechanisms. This scenario therefore solves the chicken and-egg problem of how such complicated machinery evolved, for no major shifts in import mechanism or intermolecular binding properties of the roughly 50 individual proteins comprising the import machinery need have occurred since mitochondria first evolved; molecules added early, e.g. Tim22, do not need receptors putatively added later, e.g. Tom20. Evolution of these key macromolecular complexes is apparently marked by a high degree of conservatism and stasis, despite minor improvements. Specialists will note a few partial exceptions to this that suggest some slight degree of adjustment to ‘early’ mechanisms after later ones were added, but I suggest these simply increased efficiency or rates and were not fundamental. The most important apparent exception to this generalization is Tom40 itself, which is now recognized for import (through other Tom40 pores) by Tom20 and Tom22 (). Originally it would have been coded by the proteobacterial genome, exported to the periplasm by YidC and into the OM by Omp85 (see a). Its present requirement for recognition by Tom20 and 22 does not contradict my scenario; it means only that its gene probably remained in the mitochondrion until after Tom20/22 were added to TOM. Such dependence probably could not have evolved if Tom40 came from the host, as sometimes suggested (). Overall what is striking is that one can formulate a synthesis reflecting known targeting mechanisms and interdependencies, and with selectively and mechanistically plausible intermediate stages. Further work will test the fundamental thesis in more detail and may reveal extra complications.

11/10/2017 · The mechanism of photosynthesis ..

Finally, why was protomitochondrial photosynthesis lost and respiration retained? The enslaved α-proteobacterium was probably one of many that photosynthesize only under anaerobic conditions. If the host, although a facultative anaerobe, spent at least half its life under aerobic conditions when certain photosynthesis-specific proteins were not expressed, it would be very easy for mutations abolishing photosynthesis to occur during aerobic growth without counterselection until long after they spread considerably. Loss of photosynthesis occurred repeatedly within proteobacteria and among eukaryote algae, so may need no specific explanation. However, as the host was a phagotroph—the first one—it would often have had no shortage of carbon source, for bacterial food can be superabundant. It could have benefited more from increased respiratory efficiency when aerobic than from photosynthesis when anaerobic after evolution of the AAC. Under anaerobic conditions, evolution of PFOR/hydrogenase could enable it to extract some more energy from its prey than by simple glycolysis and have the advantage over photosynthesis of working in the dark. As host and symbiont were probably both facultative aerobes, the optimal environment for the initial enslavement may have been anaerobic/aerobic interfaces or where redox conditions fluctuated unstably between them. As soon as enslavement increased respiration efficiency, its stronger oxygen sink would allow intracellular cultivation also of cyanobacteria; use of their photosynthesate under aerobic conditions could have become more synergistic with mitochondria and peroxisomes than anaerobic photosynthetic contributions by the protomitochondrion. A three way synergy between host, protomitochondrion and symbiotic cyanobacteria could have played a central role in early evolution of both mitochondria and peroxisomes, as suggested previously (); key differences from that earlier proposal are the protomitochondrion being initially photosynthetic, the delay of cyanobacterial-to-plastid conversion (by evolving protein import: ) till after the primary eukaryotic divergence into unikonts and bikonts and the autogenous, not symbiogenetic, origin of peroxisomes (). Thus fluctuating redox levels and fluctuating prey levels could have favoured these changes.

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