Capture of endogenously biotinylated proteins from Pseudomonas aeruginosa displays unexpected downregulation of LiuD upon iron nutrition.

The uptake and storage but also removal of excess iron are of utmost importance to microorganisms since surplus levels of iron may lead to the formation of reactive oxygen species. Therefore, iron homeostasis is generally tightly regulated by the ferric uptake regulator (Fur), a global iron regulator acting as a transcriptional repressor. While detecting biotinylated proteins in labelling experiments, we discovered that the endogenously biotinylated protein LiuD differentially accumulated upon iron treatment. LiuD represents the α-subunit of the methylcrotonyl‐CoA‐carboxylase (MCCase), an enzyme from the leucine/isovalerate utilization pathway. Real-time PCR transcription analysis revealed that the observed lower levels of LiuD biotinylation could be traced back to lower LiuD protein levels via a transcriptional repression of liuABCDE expression that however does not seem to be mediated by Fur. In accordance with LiuD’s role for the leucine/isovalerate utilization pathway and its protein level regulation by nutritional iron levels, we found that wild-type Pseudomonas aeruginosa did not grow in the presence of iron if the medium contained only leucine as a carbon source. Conversely, iron stimulated the growth when glucose was used as a carbon source. Our study thus demonstrates the complexities of iron-regulated bacterial growth in Pseudomonas aeruginosa.