Myxococcus xanthus, a gram-negative bacterium, has a complex life cycle. In response to starvation, most cells in a population participate in the formation of multicellular aggregates (i.e., fruiting bodies) in which cells differentiate into spores. However, some cells do not enter aggregates. In this and the two accompanying reports, the biology and physiology of these nonaggregated cells is examined. A technique to separate aggregated cells from nonaggregated cells was developed; then differentiating cells at stages throughout the course of development were isolated. In this report we (i) describe peripheral rods, those cells which remain outside aggregates after aggregation has ceased in the rest of the population; (ii) document the occurrence of peripheral rods in several wild-type strains; and (iii) characterize the expression of developmentally regulated genes in both aggregated and nonaggregated cells. These studies have shown that myxobacterial hemagglutinin, protein S (Tps), protein S1 (Ops), protein C, and several phosphatase activities are expressed in cell-type-specific patterns. These data demonstrate that peripheral rods constitute a cell type distinct from either vegetatively growing cells or spores. The description of a second, late developmental cell type (in addition to spores) opens an entirely new line of investigation in M. xanthus, i.e., the regulation of the differentiation of vegetatively growing cells into two cell types that differ significantly in biology, shape, and localization within the population.
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