algae/??/short-term-regulation-of-PS.htm
Raven (1984)
State 1- State 2 transition could have a role in balancing the redox state of the photosystems and increasing production of ATP when excess reductant is being produced
State 1 occurs in darkness or illumination conditions that over excite RC1. The cell responds by favoring the distribution toward RC2 to compensate.
Sate 2 occurs at high light or other conditions which over excite RC2. The distribution favors RC1.
e- flow can be allocated to:
ferredoxin to NADP+ (non-cyclic e- flow)
O2 (pseudocyclic electron flow - the Mehler reaction)
back to cytochrome b6/f complex (cyclic electron flow)
Falkowski and Raven (1997)
pg 91 based on low photon flux, less light is dissipated than would be expected from a pure Poisson function, postulate exitation transfer between reaction centers. An exciton might wander around until it finds an open reaction center.
Down regulation
??
Fujita et al 1994 - in Bryant
short term adaptation by state transition (minutes)
PS1:PS2 ratio changes over longer periods of time (days)
proportion of non-cyclic to cyclic may be needed to support ATP needed for ion uptake (such as HCO3-) or osmoregulation
energy distribution from PBS is called the state transition
PBS light energy goes to PS2 then PS1 by spillover
in higher plants LHC II distributes energy to PS1 and PS2 depending on its phosphorylation status
non-phosphorylated LHC II - most energy to PS2
phosphorylated LHC II - more energy to PS1
hypothesized - protein kinase is activated by reduced PQH2 (or Cytb6/f), activation causes more energy to be distributed to PS1 and balances that redox level of the PQ/PQH2 pool
dephosphorylation of LHC II is by a constitutive phosphatase - requiring constant phosphorylation to keep LHC II in state 2
could be related to redox state of Cytb6/f complex also
Kana, Geider and Critchley 1997
balanced energy flow between PS2 and PS1
redox poise of PQ pool has transcription control of cab gene which directs synthesis of antenna pigment protein LHC II
call this the regulatory ratio
pigment losses and specifically, the continual turnover of the D1 protein has been argued to be part of the normal regulatory system
model that goes to optimal energy flow between PS2 and PS1
Mehler has already been shown to be involved in regulation
Schreiber & Neubauer 1990
down regulation of PSII quantum yield is expressed by qN - non-photochemical quenching
Mehler-Peroxidase reaction, that allows non-assimilatory e- flow, is responsivle for increasing the transmembrane delta pH
when qN is increased (high transmembrane delta pH) the quantum yield is lower
one of the two major ways photosynthesis is controlled, the other being the rate limiting step at PQ to cytb6/f
mechanism for producing H2O2
Polle 1996
Mehler reaction
may increase if the utilisation of NADPH is blocked and NADP+ is not available
delta pH across membrane leads to down regulation of photosynthesis
Cohen v738
there are many sources of ROS that should occur in any cell, whether it's a cyanobacterium or not
including leakage from mitochondrial electron transport, redox cycling that includes Fe and Cu and lipid peroxidation reactions
Gonzalez-Flecha and Demple v899
summarizes hydrogen peroxide concentrations in many systems
bacteria (E. coli) and plant cells have higher steady steate amount than most mammalian cells
quinone pool is one source
thus the mechanisms that produce H2O2 and other ROS in cells are common, expect that they have similar regulatory effect (part 4)
the dual function membrane has respiration and photosynthetic processes that need to be coordinated
Obinger et al 1998
cyanobacteria were the first to make O2 and among the first to be affected by it
dual-function photosynthetic-respiratory assembly
contains thylakoid Cytb6/f and another, immunologically different, mitochondria-like multi-component dehydrogenase and cytochrome bc1 complex.
cytosolic Fe-SOD and thylakoid bound Mn-SOD help scavenge O2-
SOD in cyanobacteria respond to SO -
good second messenger
Gilbert - creation is inevitable side product of reaction and mechanisms exist to destroyed rapidly