Cell-Cell Interactions and Development in Myxococcus Xanthus

Abstract

Myxococcus xanthus is a gram-negative, rod-shaped bacterium which shows complex cell-cell interactions during vegetative growth and fruiting-body formation. In nature, Myxococcus xanthus grows and moves as a large community of food gathering and predatory organisms. Cell-cell contact is maintained throughout vegetative growth and characteristic behaviors are exhibited in the presence of certain stimuli. In addition, cell-cell stimulation can be observed in the movement of cells. Thus there must exist specific chemical signals that help direct the movement of individual cells within the community. The most dramatic behaviors exhibited by Myxococcus xanthus are associated with starvation. Starved cells aggregate and then sporulate within raised mounds. Complex biochemical changes which are part of a developmental program occur in these cells. For example, protein S first appears in the cytoplasmic fraction of cells, and is later translocated to the cell surface where it self-assembles to form a spore surface coat. Another new developmental protein that is synthesized at this time is myxobacterial hemagglutinin, a lectin that accumulates in the shockable fraction of developmental cells and that has been located in distinct patches at the cell poles. This protein may be important in end-to-end associations during cellular aggregation. Cell-cell signalling has also been found to be important during fruiting-body formation. Certain non-fruiting mutants can be stimulated to fruit by wild-type cells or by mutants from a different extracellular complementation group. Analysis of mutants has shown that the developmental program in Myxococcus xanthus appears to have two major components: one controls aggregation; the other regulates sporulation. Links exist between these two components, since some developmental mutants are defective in both functions. The study of cell-cell interactions during vegetative growth and fruiting-body formation in M. xanthus provides an opportunity to analyze complex interactions in a simple model system.