Table of Contents:
Two-component systems are widely used by bacteria as signaling modules to
sense, response and adapt to environmental changes. In Myxococcus xanthus,
two-component systems play an essential role during the complex starvation induced developmental program. During development, cells first migrate into
mounds and then, within these mounds differentiate into spores, forming
multicellular structures termed fruiting bodies. It has been previously
demonstrated that progression through the developmental program is
modulated by the RedCDEF proteins which are postulated to form an unusual
two-component signal transduction system consisting of two histidine kinase
homologs (RedC and RedE) and two response regulator homologs (RedD and
RedF) (Higgs et al, 2005).
To determine how the signals flow between these unusual two-component
signaling proteins, both genetic and biochemical approaches were employed.
Analysis of in-frame deletion and non-functional point mutants in each gene
determined that RedF in its phosphorylated state and the histidine kinase
activity of RedC are necessary to repress progression through the
developmental program, while RedE and RedD are necessary to induce
developmental progression. Genetic epistasis experiments indicated that RedE
specifically antagonizes function of RedF, and RedD acts upstream to RedE.
Our biochemical analyses demonstrate that RedC readily autophosphorylates
and the phosphoryl group can be transferred to the RedD. Interestingly, RedE
does not appear to autophosphorylate, but instead receives a phosphoryl group
from RedD. Furthermore, RedE also acts as phosphatase on RedF.
Taken together, these data suggest a model for a sophisticated signaling
system in which RedC is likely to act as kinase on RedF to repress
developmental progression. Developmental repression is relieved when RedC is
induced, by an unknown mechanism, to transfer its phosphoryl group to RedD,
which then passes the phosphoryl group to RedE. The phosphorylation of RedE
allows RedE to de-phosphorylate RedF. Thus, this work defines a novel “four component” signal transduction mechanism within the two-component signal