To survive changing environmental conditions, microbes have evolved complicated regulatory circuits to integrate the myriad signals that provide needed input for adaptation. Some of the most dramatic adaptive responses manifest as marked changes in cell motility and production of distinct morphotypes or quiescent forms. Myxococcus xanthus is a soil-dwelling, antibiotic-producing, gliding bacterium that survives in nature by preying on microbes and decaying vegetation. When nutrients are exhausted, cells build multicellular fruiting bodies that contain rod-shaped cells that differentiate into quiescent spores. We use M. xanthus as a model for the two projects described below.
- Phenotypic (phase) variation. The term phenotypic variation is used to describe the ability of microbes to alter the expression of various cellular components, such as flagella or surface proteins. Some pathogens use phase variation as a means to evade host defenses, thus allowing the microbe to survive destruction by the immune system. Less is known about phase variation in non-pathogens although preliminary evidence suggests that phase variation contributes to survival of cells in the environment. M. xanthus phase varies between a yellow (DKX pigment +), swarm proficient variant and a tan (DKX pigment -), swarm deficient variant. Yellow variants can phase to tan and vice versa. Both cell types play critical roles in survival. Using microarray analysis, we showed that genes for production of secondary metabolites, including the yellow pigment, and autolysis are expressed in the yellow phase. In contrast, tan phase cells express serine-threonine kinases, receptor proteins, and oxidative stress proteins. The expression profiles area consistent with the observed phenotypes. During growth, outward swarming yellow cells produce antibacterial and antifungal compounds that likely protect the slower, more stable tan cells. During development, lysis of yellow cells releases the yellow pigment, which stimulates sporogenesis in the tan phase cells. Hence, a novel type of symbiosis exists between yellow and tan variants. Independent student projects involve using molecular and genetic tools to elucidate the mechanism that allows M. xanthus cells to undergo this remarkable reversible switch and to test the hypothesis that the biological interplay between the two cell types has evolved as a mechanism to favor sporulation.
The mechanism of gliding movement. How does a bacterial cell coordinate the activity of two independent molecular motors to glide over different types of surfaces? We propose that a small Ras-like GTPase encoded by mglA, the only gene common to both gliding motor systems, synchronizes the two systems and regulates the amplitude of each motor depending on the type of gliding surface. To test our hypothesis, we identified and characterized protein partners for MglA and analyzed the GTPase activity of MglA. We discovered that MglA interacts with at least three other proteins – AglZ (a coiled-coil protein), MasK (a tyrosine kinase) and MglB (a GAP-like protein). Analysis of functional MglA-Yfp in actively gliding cells was used to show that MglA associates with the cytoskeleton to generate a dynamic spiral pattern. This is consistent with the pattern seen for AglZ and AglU, two other proteins known to be essential for motility. Projects for students will be aimed at expanding our knowledge of the protein complexes that are essential for the gliding motors and using fluorescence microscopy to visualize the dynamic motor complexes.