Rifampin, an antibiotic which is known to bind to and inhibit RNA polymerase, was used to probe the molecular regulation of development in Myxococcus xanthus. Rifampin-resistant mutants were screened for defects in fruiting-body formation. About 20% of the isolates in the initial screenings showed major defects in developmental aggregation or sporulation. Eleven independent mutants with wild-type growth rates and stable phenotypes were analyzed by transduction. In these strains, the

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... istant and nonfruiting phenotypes showed cotransduction frequencies equal to or greater than 99.0 to 99.9%. The RNA polymerase activities were resistant to rifampin in vitro, indicating that the RNA polymerase is altered in these strains. Although their fruiting phenotypes are heterogeneous, these strains can be divided into two classes based on the level of aggregation. The results suggest that RNA polymerase plays a significant role in the regulation of development in M. xanthus since mutations which cause no apparent changes in vegetative growth result in striking defects in fruiting-body formation.