Analysing the Cyanobacterial PipX Interaction Network Using NanoBiT Complementation in PCC7942.

The conserved cyanobacterial protein PipX is part of a complex interaction network with regulators involved in essential processes that include metabolic homeostasis and ribosome assembly. Because PipX interactions depend on the relative levels of their different partners and of the effector molecules binding to them, in vivo studies are required to understand the physiological significance and contribution of environmental factors to the regulation of PipX complexes. Here, we have used the NanoBiT complementation system to analyse the regulation of complex formation in PCC 7942 between PipX and each of its two best-characterized partners, PII and NtcA.

The Signal Transduction Protein PII Controls the Levels of the Cyanobacterial Protein PipX.

Cyanobacteria, microorganisms performing oxygenic photosynthesis, must adapt their metabolic processes to environmental challenges such as day and night changes. PipX, a unique regulatory protein from cyanobacteria, provides a mechanistic link between the signalling protein PII, a widely conserved (in bacteria and plants) transducer of carbon/nitrogen/energy richness, and the transcriptional regulator NtcA, which controls a large regulon involved in nitrogen assimilation. PipX is also involved in translational regulation through interaction with the ribosome-assembly GTPase EngA.

The ribosome assembly GTPase EngA is involved in redox signaling in cyanobacteria.

Photosynthetic organisms must cope with environmental challenges, like those imposed by the succession of days and nights or by sudden changes in light intensities, that trigger global changes in gene expression and metabolism. The photosynthesis machinery is particularly susceptible to environmental changes and adaptation to them often involves redox-sensing proteins that are the targets of reactive oxygen species generated by photosynthesis activity. Here we show that EngA, an essential GTPase and ribosome-assembly protein involved in ribosome biogenesis in bacteria and chloroplasts, also plays a role in acclimatization to environmentally relevant stress in PCC7942 and that PipX, a promiscuous regulatory protein that binds to EngA, appears to fine-tune EngA activity.

Pleiotropic effects of PipX, PipY, or RelQ overexpression on growth, cell size, photosynthesis, and polyphosphate accumulation in the cyanobacterium PCC7942.

The cyanobacterial protein PipY belongs to the Pyridoxal-phosphate (PLP)-binding proteins (PLPBP/COG0325) family of pyridoxal-phosphate-binding proteins, which are represented in all three domains of life. These proteins share a high degree of sequence conservation, appear to have purely regulatory functions, and are involved in the homeostasis of vitamin B vitamers and amino/keto acids. Intriguingly, the genomic context of the gene in cyanobacteria connects PipY with PipX, a protein involved in signaling the intracellular energy status and carbon-to-nitrogen balance.

The Conserved Family of the Pyridoxal Phosphate-Binding Protein (PLPBP) and Its Cyanobacterial Paradigm PipY.

The PLPBP family of pyridoxal phosphate-binding proteins has a high degree of sequence conservation and is represented in all three domains of life. PLPBP members, of which a few representatives have been studied in different contexts, are single-domain proteins with no known enzymatic activity that exhibit the fold type III of PLP-holoenzymes, consisting in an α/β barrel (TIM-barrel), where the PLP cofactor is solvent-exposed. Despite the constant presence of cofactor PLP (a key catalytic element in PLP enzymes), PLPBP family members appear to have purely regulatory functions affecting the homeostasis of vitamin B vitamers and amino/keto acids.

Regulatory Connections Between the Cyanobacterial Factor PipX and the Ribosome Assembly GTPase EngA.

Cyanobacteria, phototrophic organisms performing oxygenic photosynthesis, must adapt their metabolic processes to important environmental challenges, like those imposed by the succession of days and nights. Not surprisingly, certain regulatory proteins are found exclusively in this phylum. One of these unique proteins, PipX, provides a mechanistic link between signals of carbon/nitrogen and of energy, transduced by the signaling protein PII, and the control of gene expression by the global nitrogen regulator NtcA.

The default cyanobacterial linked genome: an interactive platform based on cyanobacterial linkage networks to assist functional genomics.

A database of cyanobacterial linked genomes that can be accessed through an interactive platform (https://dfgm.ua.es/genetica/investigacion/cyanobacterial_genetics/Resources.