A clear understanding of the mechanism of phase variation in Myxococcus xanthus, a typical soil bacterium, is important because preliminary data from many labs indicate that phase variation plays a critical role in development and gliding motility, two processes that are important for survival of a microorganism in the environment. The motility defect of several of our mutants is manifest only when the cells are in the yellow phase but they can escape the limitations of their defects by going to the tan phase. The bias toward the yellow variants that has been going on for over 30 years, has undermined our ability to appreciate some of the novel attributes of gliding. As we investigate this process with an eye on motility, we expect to uncover the genetic bases for the specific differences between yellow and tan cells and to examine the mechanism
of yellow-tan switching. The results we obtain will help us understand the roles that these two cell types play in motility and development. The proposed work promises to expand our understanding the molecular mechanisms underlying phase variation in a non-pathogenic bacterium and may shed new light on the origins of multiple cell-types.