In association with translation and amino acid synthesis, the nitrogen metabolism regulator protein, P-II, is also more abundant in P-starved cells (Fig. 2e). P-II is find more thought to regulate the assimilation of nitrogen as well as carbon sources on multiple levels (Osanai & Tanaka, 2007). P-II is phosphorylated in cyanobacteria and as such interacts with both a phosphatase and a kinase. However,
P-II phosphatase interaction is thought to control nitrate/nitrite assimilation, and as MED4 is unable to grow on those particular nitrogen sources (Moore et al., 2002), and that the kinase activity is reduced when, in the presence of ammonia in another cyanobacterium, Synechococcus elongatus PCC7942 (Lee et al., 1999), this particular function of P-II may well be redundant within MED4. With regard to amino acid synthesis, P-II has been shown to increase N-acetyl glutamate kinase (NAGK) activity (Maheswaran et al.,
2004), an enzyme in the arginine biosynthetic pathway, and identified in Synechococcus (Burillo et al., 2004; Heinrich et al., 2004). As MED4 is known to have NAGK, it is safe to assume that this cellular increase in P-II will have a constitutive affect on arginine biosynthesis. In addition to this, P-II directly influences nitrogen-related SP600125 mw gene transcription (Paz-Yepes et al., 2003), but this process is, as yet, unknown. An intriguing result is the increased abundance of the periplasmic protein, FKBP-type peptidyl-prolyl cis–trans isomerase (PPIase) (Fig. 2d), which
assists in the accelerated and correct folding of proteins bound for extracellular use (Lang et al., 1987; Lang & Schmid, 1988). This result is Flavopiridol (Alvocidib) interesting if considered in parallel with the significant increase in a membrane-associated protease (PMM0516, Fig. 2e), which would assist in recycling misfolded periplasmic proteins, and the significant increase in PhoA concentrations reported above. However, PPIase transcripts were found to be downregulated in WH8102 (Tetu et al., 2009), but this could indicate a strain-specific response to P starvation, particularly when considering the increased abundance of the MED4-specific protein PMM1416. Fatty acid biosynthesis is also detrimentally affected by P starvation. Two proteins essential in this process, acyl carrier protein (acpP) and enoyl-(acyl carrier protein) reductase (fabL), were less abundant than the control (Fig. 2e). Fatty acids have multiple intracellular uses, notably fuel storage and membrane manufacture. It could easily be deduced that with a paucity of bioavailable P, phospholipid biosynthesis and hence membrane manufacture, would be reduced. However, it is known that <1% of inducted Pi is incorporated into membranes, representing a small fraction of the cellular quota for P, and there is no evidence, as yet, for P regulation within the lipid membrane of MED4 (Van Mooy et al., 2006).