Myxococcus xanthus has a lifecycle characterized by several social interactions. In the presence of prey, M. xanthus is a predator forming cooperatively feeding colonies, and in the absence of nutrients, M. xanthus cells interact to form multicellular, spore-filled fruiting bodies. Formation of both cellular patterns depends on extracellular functions including the extracellular matrix and intercellular signals. Interestingly, the formation of these patterns also depends on several activities that involve direct cell-cell contacts between M. xanthus cells or direct contacts between M. xanthus cells and the substratum, suggesting that M. xanthus cells have a marked ability to distinguish self from nonself. Genome-wide analyses of the M. xanthus genome reveal a large potential for protein secretion. Myxococcus xanthus harbours all protein secretion systems required for translocation of unfolded and folded proteins across the cytoplasmic membrane and an intact type II secretion system. Moreover, M. xanthus contains 60 ATP-binding cassette transporters, two degenerate type III secretion systems, both of which lack the parts in the outer membrane and the needle structure, and an intact type VI secretion system for one-step translocation of proteins across the cell envelope. Also, analyses of the M. xanthus proteome reveal a large protein secretion potential including many proteins of unknown function.

Generally, the second messenger bis-(3’-5’)-cyclic dimeric GMP (c-di-GMP) regulates the switch between motile and sessile lifestyles in bacteria. Here, we show that c-di-GMP is an essential regulator of multicellular development in the social bacterium Myxococcus xanthus. In response to starvation, M. xanthus initiates a developmental program that culminates in formation of spore-filled fruiting bodies. We show that c-di-GMP accumulates at elevated levels during development and that this increase is essential for completion of development whereas excess c-di-GMP does not interfere with development. MXAN3735 (renamed DmxB) is identified as a diguanylate cyclase that only functions during development and is responsible for this increased c-di-GMP accumulation. DmxB synthesis is induced in response to starvation, thereby restricting DmxB activity to development. DmxB is essential for development and functions downstream of the Dif chemosensory system to stimulate exopolysaccharide accumulation by inducing transcription of a subset of the genes encoding proteins involved in exopolysaccharide synthesis. The developmental defects in the dmxB mutant are non-cell autonomous and rescued by co-development with a strain proficient in exopolysaccharide synthesis, suggesting reduced exopolysaccharide accumulation as the causative defect in this mutant. The NtrC-like transcriptional regulator EpsI/Nla24, which is required for exopolysaccharide accumulation, is identified as a c-di-GMP receptor, and thus a putative target for DmxB generated c-di-GMP. Because DmxB can be—at least partially—functionally replaced by a heterologous diguanylate cyclase, these results altogether suggest a model in which a minimum threshold level of c-di-GMP is essential for the successful completion of multicellular development in M. xanthus.

The nucleotide-based second messenger bis-(3′-5′)-cyclic dimeric GMP (c-di-GMP) is involved in regulating a plethora of processes in bacteria that are typically associated with lifestyle changes. Myxococcus xanthus undergoes major lifestyle changes in response to nutrient availability, with the formation of spreading colonies in the presence of nutrients and spore-filled fruiting bodies in the absence of nutrients. Here, we investigated the function of c-di-GMP in M. xanthus and show that this bacterium synthesizes c-di-GMP during growth. Manipulation of the c-di-GMP level by expression of either an active, heterologous diguanylate cyclase or an active, heterologous phosphodiesterase correlated with defects in type IV pilus (T4P)-dependent motility, whereas gliding motility was unaffected. An increased level of c-di-GMP correlated with reduced transcription of the pilA gene (which encodes the major pilin of T4P), reduced the assembly of T4P, and altered cell agglutination, whereas a decreased c-di-GMP level correlated with altered cell agglutination. The systematic inactivation of the 24 genes in M. xanthus encoding proteins containing GGDEF, EAL, or HD-GYP domains, which are associated with c-di-GMP synthesis, degradation, or binding, identified three genes encoding proteins important for T4P-dependent motility, whereas all mutants had normal gliding motility. Purified DmxA had diguanylate cyclase activity, whereas the hybrid histidine protein kinases TmoK and SgmT, each of which contains a GGDEF domain, did not have diguanylate cyclase activity. These results demonstrate that c-di-GMP is important for T4P-dependent motility in M. xanthus.IMPORTANCE We provide the first direct evidence that M. xanthus synthesizes c-di-GMP and demonstrate that c-di-GMP is important for T4P-dependent motility, whereas we did not obtain evidence that c-di-GMP regulates gliding motility. The data presented uncovered a novel mechanism for regulation of T4P-dependent motility, in which increased levels of c-di-GMP inhibit transcription of the pilA gene (which encodes the major pilin of T4P), ultimately resulting in the reduced assembly of T4P. Moreover, we identified an enzymatically active diguanylate cyclase that is important for T4P-dependent motility.

In Myxococcus xanthus the extracellular matrix is essential for type IV pili-dependent motility and starvation-induced fruiting body formation. Proteins of two-component systems including the orphan DNA binding response regulator DigR are essential in regulating the composition of the extracellular matrix. We identify the orphan hybrid histidine kinase SgmT as the partner kinase of DigR. In addition to kinase and receiver domains, SgmT consists of an N-terminal GAF domain and a C-terminal GGDEF domain. The GAF domain is the primary sensor domain. The GGDEF domain binds the second messenger bis-(3′-5′)-cyclic-dimeric-GMP (c-di-GMP) and functions as a c-di-GMP receptor to spatially sequester SgmT. We identify the DigR binding site in the promoter of the fibA gene, which encodes an abundant extracellular matrix metalloprotease. Whole-genome expression profiling experiments in combination with the identified DigR binding site allowed the identification of the DigR regulon and suggests that SgmT/DigR regulates the expression of genes for secreted proteins and enzymes involved in secondary metabolite synthesis. We suggest that SgmT/DigR regulates extracellular matrix composition and that SgmT activity is regulated by two sensor domains with ligand binding to the GAF domain resulting in SgmT activation and c-di-GMP binding to the GGDEF domain resulting in spatial sequestration of SgmT.

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