Chapter 11 : Developmental Aggregation and Fruiting Body Formation in the Gliding Bacterium Myxococcus xanthus

Gene organizations within motility- and directed-motility-associated loci. (A) dif locus (GenBank accession no. AF076485); (B) frz locus (accession no. AF049107, U47814, J04157, M35192, M35200, and U44437); (C) abcA locus (accession no. AF047554); (D) pil locus (accession no. AF003632, L39904, and L78131). Genes which when mutated result in the frizzy aggregation phenotype during development are shown in dark gray. Restriction enzyme sites: B, BamHI; C, ClaI (shown for the pil region only); P, PstI; and S, SacI. Arrows designate direction of transcription of genes.

Information flow through the enteric chemotaxis system. Chemical stimuli are sensed by the transmembrane MCP receptor proteins. Excitation signals are relayed via CheW to the histidine protein kinase CheA. CheA can both autophosphorylate and, when stimulated by the MCPs, phosphorylate CheY. CheY-P then interacts with the switch proteins of the flagellar motor, regulating the direction of flagellar rotation. Dephosphorylation of CheY-P is stimulated by CheZ, while adaptation to stimuli requires methylation and demethylation of the MCPs by CheR and CheB, respectively.

Alignment of CheY-like proteins from M. xanthus with CheY from S. typhimurium. (a) Alignment of the M. xanthus CheY homologies in the DifD ( Yang et al., 1998b ), FrzE ( McCleary and Zusman, 1990 ), and FrzZ ( Trudeau et al., 1996 ) proteins against S. typhimurium CheY ( Stock et al., 1985 ). The phosphorylation site (S. typhimurium Asp57) and associated amino acids are marked with “*.” Amino acids proposed to be involved with switch protein interactions are indicated with “o” (b) Dendrogram created from the CheY protein alignment.

Wild-type and mutant developmental-aggregation phenotypes. (a) Strain DZ2 (wild type), shown developing at high cell numbers (2 × 1010 CFU/ml) after 48 h of incubation at 34°C; (b) SW505 (difA; signal transduction mutant), showing formation of only small mounds after 48 h of incubation; (c) DK2630 (csgA; extracellular-signaling mutant), showing formation of only small mounds after 24 h of development; (d) DZF3558 ( ΔfrzA-E; signal transduction mutant), showing the characteristic frizzy phenotype in the strain FB background after 72 h of incubation; (e) DZF1 (rpoE1; ECF sigma factor mutant), showing trails of aggregates rather than discrete mounds after 24 h of development; (f) DZF1 (espA; signal transduction mutant), showing many small, closely spaced aggregates (photo courtesy of K. Cho); (g) DZ2 (tagE; temperature-dependent aggregation mutant), showing no aggregation at 34°C (provided by K. O'Connor); (h) DZ2 (tagE) showing wild-type aggregation at 28°C (provided by K. O'Connor). All cells were plated on CF starvation agar ( Hagen et al., 1978 ).

Model showing the proposed interactions within the Frz signal transduction pathway during developmental aggregation. Components of the pathway that when mutated result in cells displaying the frizzy aggregation phenotype are colored dark gray. The FrzZ and AbcA proteins are suggested to act upstream of the central components, FrzCD, FrzA, and FrzE, and may regulate the export of a developmentally important molecule. This molecule might be modified or might modify a signal associated with late developmental aggregation. The histidine protein kinase (HPK) that phosphorylates FrzZ is unknown. However, FrzZ is proposed to potentially interact with the RpoEl protein to regulate gene expression. Inputs to the FrzCD receptor may be associated with the C-signaling pathway as well as with the AbcA-associated pathway. Other, unknown inputs may also occur. The role of FrzB is unknown, although the protein is suggested to interact with FrzCD. The FrzF and FrzG proteins methylate and demethylate FrzCD. It is also suggested that FrzF may interact with an unknown regulatory protein. The FrzS protein may be phosphorylated by FrzE and interact directly with the S-motility system. Likely pathways or interactions are shown connected by black arrows. More speculative interactions are shown connected by gray arrows.

Electron micrograph of an M. xanthus cell showing the polar associated type IV pili. A strain DZ2 (wild-type) cell is shown negatively stained with uranyl acetate.