Abstract
Synechococcus elongatus PCC 7942 is a paradigmatic model organism for nitrogen regulation in cyanobacteria. Expression of genes involved in nitrogen assimilation is positively regulated by the 2-oxoglutarate receptor and global transcriptional regulator NtcA. Maximal activation requires the subsequent binding of the co-activator PipX. PII, a protein found in all three domains of life as an integrator of signals of the nitrogen and carbon balance, binds to PipX to counteract NtcA activity at low 2-oxoglutarate levels. PII-PipX complexes can also bind to the transcriptional regulator PlmA, whose regulon remains unknown. Here we expand the nitrogen regulatory network to PipY, encoded by the bicistronic operon pipXY in S. elongatus. Work with PipY, the cyanobacterial member of the widespread family of COG0325 proteins, confirms the conserved roles in vitamin B6 and amino/keto acid homeostasis and reveals new PLP-related phenotypes, including sensitivity to antibiotics targeting essential PLP-holoenzymes or synthetic lethality with cysK. In addition, the related phenotypes of pipY and pipX mutants are consistent with genetic interactions in the contexts of survival to PLP-targeting antibiotics and transcriptional regulation. We also showed that PipY overexpression increased the length of S. elongatus cells. Taken together, our results support a universal regulatory role for COG0325 proteins, paving the way to a better understanding of these proteins and of their connections with other biological processes.
Highlights
Cyanobacteria are phototrophic organisms that perform oxygenic photosynthesis and require the assimilation of ammonia for autotrophic growth
The genomic information indicates that co-expression and even translational coupling between pipY with cyanobacterial-exclusive (pipX) and pipY may be relatively frequent amongst cyanobacteria, suggesting a strong functional connection between PipX and PipY in this phylogenetic group
Once we obtained in vivo evidence indicating that PipY is a typical COG0325 protein, functioning in amino/keto acid and vitamin B6 homeostasis, we explored the PipY connection with the nitrogen regulator PipX in the context of the susceptibility to pyridoxal phosphate (PLP)-targeting antibiotics
Summary
Cyanobacteria are phototrophic organisms that perform oxygenic photosynthesis and require the assimilation of ammonia for autotrophic growth. The same principle can be applied to proteins encoded within the same operon, in the cyanobacterium Synechococcus elongatus PCC7942 (hereafter S. elongatus), where most of the mRNA transcripts identified (approximately 62%) are monocistronic and the co-transcription of 2 given genes provides a very strong suggestion of functional association (Vijayan et al, 2011; Memon et al, 2013). This is the case of pipX and its downstream gene Synpcc7942_2060, called hereafter pipY in reference to its linkage to PipX in cyanobacteria
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