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The extracellular matrix of Myxococcus xanthus is essential for type IV pili-dependent motility and starvation-induced fruiting body formation. The composition of the extracellular matrix is regulated by proteins of two-component systems including the orphan response regulator DigR.
In this work I identified and characterized the orphan hybrid histidine kinase SgmT as corresponding partner of DigR. SgmT contains an N-terminal GAF domain and a C-terminal GGDEF domain. Using genetical and biochemical analyses the function of each domain was addressed. I found that the GAF domain is the primary sensor domain, while the GGDEF domain binds the second messenger bis-(3′-5′)-cyclic-dimeric-GMP (c-di-GMP) and functions as a receptor to spatially sequester SgmT. In addition, a DigR binding site has been identified in the promoter of the fibA gene, encoding for an abundant extracellular matrix-associated zinc-metalloprotease. Comparative expression studies in combination with the identified DigR binding site allowed the revelation of the SgmT/DigR regulon-containing genes, encoding for secreted proteins and enzymes involved in secondary metabolite synthesis. These findings strongly indicate that: 1.) SgmT/DigR regulates the composition of the extracellular matrix and 2.) SgmT activity is regulated by two sensor domains with ligand binding to the GAF domain resulting in SgmT kinase activation and c-di-GMP binding to the GGDEF domain resulting in spatial sequestration of SgmT.
Additionally, conserved c-di-GMP-dependent proteins were identified in M. xanthus and I started to investigate their function in important cellular processes. Preliminary data show that c-di-GMP participates in the regulation of the starvation-induced developmental program of M. xanthus.