Interaction network in cyanobacterial nitrogen regulation: PipX, a protein that interacts in a 2-oxoglutarate dependent manner with PII and NtcA

Authors

  • Javier Espinosa,

    1. División de Genética, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain.
    2. Institut für Mikrobiologie und Molekularbiologie, Universität Giessen, Heinrich-Buff-Ring 26-32, D 35392 Giessen, Germany.
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  • Karl Forchhammer,

    1. Institut für Mikrobiologie und Molekularbiologie, Universität Giessen, Heinrich-Buff-Ring 26-32, D 35392 Giessen, Germany.
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  • Sergio Burillo,

    1. División de Genética, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain.
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  • Asunción Contreras

    Corresponding author
    1. División de Genética, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain.
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*E-mail contrera@ua.es; Tel. (+34) 96 590 3957; Fax (+34) 96 590 9569.

Summary

Cyanobacteria perceive nitrogen status by sensing intracellular 2-oxoglutarate levels. The global nitrogen transcription factor NtcA and the signal transduction protein PII are both involved in 2-oxoglutarate sensing. PII proteins, probably the most conserved signal transduction proteins in nature, are remarkable for their ability to interact with very diverse protein targets in different systems. Despite widespread efforts to understand nitrogen signalling in cyanobacteria, the involvement of PII in the regulation of transcription activation by NtcA remains enigmatic. Here we show that PipX, a protein only present in cyanobacteria, interacts with both PII and NtcA and provides a mechanistic link between these two factors. A variety of in vivo and in vitro approaches were used to study PipX and its interactions with PII and NtcA. 2-Oxoglutarate favours complex formation between PipX and NtcA, but impairs binding to PII, suggesting that partner swapping between these nitrogen regulators is driven by the 2-oxoglutarate concentration. PipX is required for NtcA-dependent transcriptional activation in vivo, thus implying that PipX may function as a prokaryotic transcriptional coactivator.

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