GUEST COMMENTARY Bacterial Development in the Fast Lane

Timing is everything—in love, in war, in sports, and in politics—but what about during multicellular development of Myxococcus xanthus bacteria? In this issue, Higgs et al. (9) report that loss of a single protein kinase can radically change the timing of aggregation and sporulation of 100,000 cells as they race to build a fruiting body when faced with starvation. Key transcription factors accumulate earlier in the kinase mutant, accelerating subsequent developmental events. What are the consequences of development in the fast lane? Spores form in normal numbers, but in smaller or deformed fruiting bodies or outside of fruiting bodies. Timing is not everything, but it probably has a profound impact on the ecology and evolution of social bacteria. M. xanthus is the most studied of the myxobacteria, a group of gram-negative, deltaproteobacteria that move by gliding and inhabit soils globally (32). They are predators in the microbial world, hunting in packs and secreting antibiotics and enzymes to kill and devour their prey. This cooperative feeding behavior presumably provided selective pressure to evolve multicellular development. When food becomes scarce, cells in a swarm of M. xanthus aggregate into mounds, each containing about 100,000 cells (Fig. 1). In the nascent fruiting body, rod-shaped cells differentiate into spherical spores. Outside of fruiting bodies, some cells remain as peripheral rods. During the aggregation phase, many cells undergo programmed cell death, perhaps facilitating formation of spores and persistence of peripheral rods. The mature fruiting body is a cohesive, albeit dormant, population, capable of being transported to a new environment, where prey or other nutrient sources trigger germination of the spores, unleashing a swarm of predators. The pace of fruiting body development is influenced by a mind-boggling number of genetic and environmental factors. Higgs et al. (9) focused on a mutant with a somewhat unusual but by no means unique ability to aggregate and sporulate faster than normal. This mutant was previously identified because it made smaller and more numerous fruiting bodies than the wild type under certain conditions (2). The mutant was shown to have a loss-of-function mutation in espA (early sporulation). EspA was predicted to encode a histidine protein kinase and was hypothesized to inhibit sporulation during the aggregation phase of development. Higgs et al. (9) confirm the prediction that EspA is a histidine protein kinase. They also show that after autophosphorylation the kinase very likely transfers the phosphoryl group to its C-terminal receiver domain. What happens after that is unknown. Also unknown is the signal(s) that presumably stimulates EspA autophosphorylation. EspA’s N-terminal sensor domain is complex. It contains a Forkhead-associated domain that is thought to interact with at least PktA5 and/or PktB8, two adjacently encoded serine/threonine protein kinases, and two PAS/PAC domains that might sense the redox potential of the cell (2, 29). The espA gene is cotranscribed with espB, and the operon is induced very early during development (2, 9). EspB is a predicted membrane protein that appears to antagonize EspA (2). EspA is a cytoplasmic kinase that appears to function within a complex signal transduction network. Since no predicted response regulator is encoded nearby in the genome, EspA is an orphan kinase whose output is unknown. Despite not knowing EspA’s input or output, Higgs et al. (9) sought a clue about its output by examining molecular markers of development in the espA-null mutant. By identifying the earliest change in the developmental program, the authors could begin to understand which step is regulated by EspA to control the timing of development. Molecular events that drive M. xanthus development have been revealed by classical and molecular genetic approaches, as well as biochemical and, recently, genomic approaches (Fig. 1). Nutrient limitation triggers production of the intracellular signal (p)ppGpp and expression of early developmental genes (6, 26). The (p)ppGpp signal leads to elaboration of the extracellular A-signal, a mixture of amino acids and peptides used to sense population density (11, 16). If sufficiently high, A-signaling generates the next wave of gene expression and is necessary for aggregation to begin. Also needed for aggregation is a cascade of transcription factors that includes several enhancer-binding proteins (N. B. Caberoy and A. G. Garza, personal communication), MrpC, and FruA (reviewed in reference 13). All of these transcription factors likely respond to signals, but the signals are unknown, with one exception. FruA is believed to be phosphorylated in response to C-signaling from another cell (3). This signaling requires CsgA and possibly end-to-end contact of cells (reviewed in reference 27). As cells increasingly make contacts during aggregation, it has been proposed that increased C-signaling leads to a high level of phosphorylated FruA that activates expression of genes required for sporulation. According to this model, C-signaling coordinates aggregation with sporulation, ensuring that spores form within fruiting bodies. Higgs et al. (9) discovered that the fruA transcript and FruA protein concentrations are elevated in the espA mutant compared with the wild type during aggregation. Genes required * Mailing address: Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824. Phone: (517) 355-9726. Fax: (517) 353-9334. E-mail: [email protected]. Published ahead of print on 9 May 2008.