USING A PHASE-LOCKED MUTANT OF MYXOCOCCUS-XANTHUS TO STUDY THE ROLE OF PHASE VARIATION IN DEVELOPMENT

The bacterium Myxococcus xanthus undergoes a primitive developmental c ycle in response to nutrient deprivation. The cells aggregate to form fruiting bodies in which a portion of the cells differentiate into env ironmentally resistant myxospores. During the growth portion of the M. xanthus life cycle, the organism also undergoes a phase variation, in which cells alternate between yellow and tan colony-forming variants. Phase variation occurs in our laboratory strain (M102, a derivative o f DK1622) at a frequency high enough that a single colony of either th e yellow or the tan phase already contains cells of the alternate phas e. In this study we demonstrate that tan cells within a predominantly yellow population of phase variation-proficient cells are preferential ly recovered as heat- and sonication-resistant spores. To further inve stigate the possibility of a differential role of tan and yellow cells during development, a tan-phase-locked mutant was used to compare the developmental phenotypes of a pure tan population with a predominantl y yellow, phase variation-proficient population. Pure tan-phase popula tions did not produce fruiting bodies or mature spores under condition s in which predominantly yellow wild-type populations did so efficient ly. Pure populations of tan-phase cells responded to developmental ind uction by changing from vegetative rod-shaped cells to round forms but were unable to complete the maturation to heat- and sonication-resist ant, refractile spores. The developmental defect of a tan-phase-locked mutant was rescued by the addition of phase variation-proficient cell s from a predominantly yellow culture. In such mixtures the tan-phase- locked mutant not only completed the process of forming spores but als o was again preferentially represented among the viable spores. These findings suggest the intriguing possibility that the tan-phase cells w ithin the vegetative population entering development are the progenito rs of spores and implicate a requirement for yellow-phase cells in spo re maturation.