Extracellular and intracellular determination of light-induced potential changes during photophobic reactions in blue-green algae

The cyanobacterial taxis protein HmpF regulates type IV pilus activity in response to light

Motility is ubiquitous in prokaryotic organisms including the photosynthetic cyanobacteria where surface motility powered by type 4 pili (T4P) is common and facilitates phototaxis to seek out favorable light environments. In cyanobacteria, chemotaxis-like systems are known to regulate motility and phototaxis. The characterized phototaxis systems rely on methyl-accepting chemotaxis proteins containing bilin-binding GAF domains capable of directly sensing light, and the mechanism by which they regulate the T4P is largely undefined. In this study we demonstrate that cyanobacteria possess a second, GAF-independent, means of sensing light to regulate motility and provide insight into how a chemotaxis-like system regulates the T4P motors. A combination of genetic, cytological, and protein-protein interaction analyses, along with experiments using the proton ionophore carbonyl cyanide m-chlorophenyl hydrazine, indicate that the Hmp chemotaxis-like system of the model filamentous cyanobacterium Nostoc punctiforme is capable of sensing light indirectly, possibly via alterations in proton motive force, and modulates direct interaction between the cyanobacterial taxis protein HmpF, and Hfq, PilT1, and PilT2 to regulate the T4P motors. Given that the Hmp system is widely conserved in cyanobacteria, and the finding from this study that orthologs of HmpF and T4P proteins from the distantly related model unicellular cyanobacterium Synechocystis sp. strain PCC6803 interact in a similar manner to their N. punctiforme counterparts, it is likely that this represents a ubiquitous means of regulating motility in response to light in cyanobacteria.

Fluorescent probes for non-invasive bioenergetic studies of whole cyanobacterial cells

Fluorescent DeltapH and DeltaPsi indicators have been screened for the non-invasive monitoring of bioenergetic processes in whole cells of the cyanobacterium Synechocystis sp. PCC 6803. Acridine yellow and Acridine orange proved to be the best DeltapH indicators for the investigation of thylakoid and cytoplasmic membrane energization: While Acridine yellow indicated only cytosolic energization, Acridine orange showed signals from both the thylakoid lumen and the cytosol that could be separated kinetically. Both indicators were applied successfully to monitor cellular energetics, such as the interplay of linear and cyclic photosynthetic electron transport, osmotic adaptation and solute transport across the cytoplasmic membrane. In contrast, useful membrane potential indicators were more difficult to find, with Di-4-ANEPPS and Brilliant cresyl blue being the only promising candidates for further studies. Finally, Acridine yellow and Acridine orange could also be applied successfully for the thermophilic cyanobacterium Synechococcus elongatus. Different from Synechocystis sp. PCC 6803, where both respiration and ATP hydrolysis could be utilized for cytoplasmic membrane energization, proton extrusion at the cytoplasmic membrane in Synechococcus elongatus was preferentially driven by ATP hydrolysis.

Calcium is required for swimming by the nonflagellated cyanobacterium Synechococcus strain WH8113

The marine cyanobacterium Synechococcus strain WH8113 swims in the absence of any recognizable organelles of locomotion. We have found that calcium is required for this motility. Cells deprived of calcium stopped swimming, while addition of calcium completely restored motility. No other divalent ions tested could replace calcium. Terbium, a lanthanide ion, blocked motility even when calcium was present at 10(5)-fold-higher concentrations, presumably by occupying calcium binding sites. Calcium chelators, EGTA or EDTA, blocked motility, even when calcium was present at 25-fold-higher concentrations, presumably by acting as calcium ionophores. Finally, motility was blocked by verapamil and nitrendipine, molecules known to block voltage-gated calcium channels of eukaryotic cells by an allosteric mechanism. These results suggest that a calcium potential is involved in the mechanism of motility.

Photoperception and photomovement

The photoperception systems in photomovement show a great diversity at several levels, (i) Different types of photomovement (namely photokinesis, photophobic response, phototaxis) make use of completely different perception systems even in the same organism, (ii) In different organisms a given type of photomovement is mediated by different and unrelated systems, as is shown in this paper particularly for phototaxis, (iii) In certain cases, a single response in one organism can be controlled by two separate and independent photoperception systems, which may even be located in different compartments. This diversity sets photomovement apart from photosynthesis and vision, which are highly uniform and stable in evolution. Possible consequences for considering the evolution are discussed.

Signal perception and amplification in photoresponses of cyanobacteria

In addition to a number of other external stimuli, the gliding cyanobacterium, Phormidium uncinatum, uses light as a clue to control movement in order to find and stay in a microhabitat with optimal conditions for growth and survival of the population. Of the three photoresponses developed in cyanobacteria, phototaxis, photokinesis and photophobic responses, the latter are the most prominent for this task. Step-down phobic responses are mediated by the photosynthetic pigments and are linked to the linear electron-transport chain. A change in the fluence rate is reflected by a change in the proton-motive force which is amplifi.

Lateral transport of energy along the coupling membranes of cyanobacteria

The light-induced electric potential changes brought about local illumination of trichomes of cyanobacteria Phormidium uncinatum have been studied by means of extracellular electrodes. Responses of several electrodes located at various distances from the illuminated area of the trichome were monitored simultaneously. They turned out to be similar in shape: a rapid increase to the maximum value was followed by a slow decay toward a nonzero residual level. The results offer strong evidence in favor of power transmission along the trichome. The computerized experimental data lend support to the notion of a unified system of coupling membranes acting as a passive cable for electrical propagation, the cable parameters being tau C = 440 sec cm-2 and gamma = 0.07 cm.

Electrical nature of the taxis signal in cyanobacteria

Electrical events after a light-dark stimulus were studied in the multicellular organism Phormidium uncinatum. Normally, such a stimulus causes the gliding trichome to reverse direction. By directing a large light spot on the end of a batch of trichomes and then switching it off, we achieved synchronization of the trichomes, since the "head" is much more sensitive than the "tail." The abrupt disappearance of a uniform light produced a depolarization wave which initiated at the head, as registered by externally applied electrodes. The second stimulus produced a depolarization of the opposite direction, reflecting the reorientation of the trichomes. No electrical response was observed at Ca2+ concentrations less than or equal to 10(-8) M. Factors causing oscillatory reversals, i.e., a combination of Ca2+ and A23187, or a viscous environment also abolished the electrical signal. Changes in an externally applied electrical field (4 V/cm2) had little effect on the motile behavior of P. uncinatum or Oscillatoria princeps. However, in the presence of 5 microM Ca2+-1 microM A23187, all the trichomes reversed synchronously to the anode after a change in polarity of an externally applied electrical field. We suggest that an increased Ca2+ concentration together with a change in delta psi (or delta mu H+) represents the taxis signal in cyanobacteria.