Figure 9

Proposed model for the regulation of rhizoferrin levels via the cAMP-PKA pathway in M. lusitanicus. Presence of organic nitrogen (peptone) or high levels of glucose possibly stimulated the enzymatic activity of adenylyl cyclase (Cyr1) and inhibited phosphodiesterase (Pde2), eventually inducing the generation of high levels of cAMP, which binds to PkaR1 (regulatory subunit 1) of PKA (protein kinase A), thereby activating catalytic PKA (PkaC unknown). This stimulates fermentative metabolism and decreases oxidative metabolism, which negatively regulates reactive oxygen species (ROS) production and decreases the mRNA levels of Rfs, probably via the transcriptional regulation of SreA and Yap1. Lower oxidative metabolism correlates with the downregulation of citrate synthase 1 (cit1) and ornithine decarboxylase (spe1), which synthetize Rfs substrates. This leads to lower levels of rhizoferrin accumulation in the cell exterior, correlating with decreased virulence. In the same context, excess Fe²⁺ led to the transcriptional downregulation of rfs, which correlated with lower virulence. Rhizoferrin chelates extracellular iron and transports it to the intracellular space; intracellular rhizoferrin could store iron and help avoid free iron toxicity. Meanwhile, decreased cAMP, low levels of glucose, presence of oxidative stressors, such as H₂O_2, or stimulation of the activity of Pde2 produced the contrary effect, inducing oxidative metabolism and increasing rhizoferrin production, which correlates with increased virulence of M. lusitanicus.