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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 cyanobacteriumNostoc punctiformeis 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 cyanobacteriumSynechocystissp. strain PCC6803 interact in a similar manner to theirN. punctiformecounterparts, it is likely that this represents a ubiquitous means of regulating motility in response to light in cyanobacteria.
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Phototaxis in a wild isolate of the cyanobacterium Synechococcus elongatus
Many cyanobacteria, which use light as an energy source via photosynthesis, have evolved the ability to guide their movement toward or away from a light source. This process, termed “phototaxis,” enables organisms to localize in optimal light environments for improved growth and fitness. Mechanisms of phototaxis have been studied in the coccoid cyanobacteriumSynechocystissp. strain PCC 6803, but the rod-shapedSynechococcus elongatusPCC 7942, studied for circadian rhythms and metabolic engineering, has no phototactic motility. In this study we report a recent environmental isolate ofS. elongatus, the strain UTEX 3055, whose genome is 98.5% identical to that of PCC 7942 but which is motile and phototactic. A six-gene operon encoding chemotaxis-like proteins was confirmed to be involved in phototaxis. Environmental light signals are perceived by a cyanobacteriochrome, PixJSe(Synpcc7942_0858), which carries five GAF domains that are responsive to blue/green light and resemble those of PixJ fromSynechocystis. Plate-based phototaxis assays indicate that UTEX 3055 uses PixJSeto sense blue and green light. Mutation of conserved functional cysteine residues in different GAF domains indicates that PixJSecontrols both positive and negative phototaxis, in contrast to the multiple proteins that are employed for implementing bidirectional phototaxis inSynechocystis.
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- Award ID(s):
- 1755220
- PAR ID:
- 10081522
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 115
- Issue:
- 52
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- p. E12378-E12387
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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