Use of recombination techniques to examine the structure of the csg locus of Myxococcus xanthus

The myxobacteria are among the simplest organisms with a developmental cycle that is dependent on cell cooperation, and they provide an outstanding system with which to study genes involved in cell interactions. Myxococcus xanthus cells which acquire a csg mutation (formerly known as spoC) lose three different traits, the ability to sporulate, the ability to stimulate adjacent Csg cells to sporulate, and the ability to ripple. The boundaries of the csg locus were determined by transferring a recombinant DNA molecule containing all or part of the locus to Csg mutants and examining the sporulation and rippling phenotypes of the transductants. Three methods were used to integrate the csg locus into the chromosome. First, the entire molecule was integrated into the chromosome by a single homologous crossover. Second, a portion of the molecule was integrated into the chromosome by two flanking homologous crossovers. Third, the entire molecule was integrated into the chromosome by site-specific recombination at a bacteriophage attachment site. Together, these techniques suggested that all of the functions of the csg locus are carried on a DNA fragment of 1.9 kbp or less. The locus appears to contain two smaller units of function. Transposon insertions or deletions in the right end of the locus disrupted sporulation and intercellular complementation of Csg mutants for sporulation, but did not disrupt rippling. The intercellular complementation of Csg mutants may reflect a natural and necessary step in the sporulation of wild-type cells, since the ability to sporulate and the ability to stimulate Csg mutants to sporulate were inseparable by any of these methods.