The perception and regulation of the cellular content of nitrogen (N) and carbon (C) is essential for all living organisms. Though the superfamily of PII proteins that are highly conserved in bacteria, archaea, algae and plants, ought to play a crucial role. It is assumed from studies in bacteria that PII is a sensor of the nitrogen and carbon status Although the function of PII in bacteria is well characterised (Forchhammer and Selim, 2019), little is known about these function in plants and especially in A. thaliana.
To understand the function of A. thaliana PII, I focussed on expression, localization, and interaction studies. Here I show that PII mRNA expression changes dependent on inorganic nitrogen under constant light. Physiological studies on PII knockdown mutants, and in addition with PII overexpression lines, revealed no evidence for the deregulation of the C/N-metabolism. This observation let appear a central role of the PII-protein for these processes in plants less probable.
Furthermore, I could show that PII localizes to foci in plastids and in extraplastidic vesicle-like structures, which could be related to protein turnover. Further, I could exclude that PII localizes to nucleoids and starch granules. I could show that A. thaliana PII co-localizes not only with the already known interaction partners N-acetyl-L-glutamate kinase (NAGK), the biotin carboxyl carrier protein 1 (BCCP1) and with the biotin/lipoyl attachment domain containing (BADC) proteins BADC2/3 in plastids but in addition with Granule-bound Starch Synthase I (GBSSI), D-Amino Acid Transaminase 1 (DAT1), one of the small Rubisco subunits RBCS3B, and Deoxyxylulose synthase (DXS), Deoxyxylulose reductoisomerase (DXR) and partially with the Geranylgeranyl diphosphate synthase 11 (GGPPS11). Using FRET-FLIM and BiFC studies, even an interaction of PII with itself, NAGK, BCCP1, GBSSI, DAT1, RBCS3B, DXS and DXR could be observed.
The localization pattern and interactions observed for A. thaliana PII indicate further unknown regulatory functions of PII in A. thaliana
