No abstract

Many bacteria can rapidly traverse surfaces from which they are extracting nutrient for growth. They generate flat, spreading colonies, called swarms because they resemble swarms of insects. We seek to understand how members of any dense swarm spread efficiently while being able to perceive and interfere minimally with the motion of others. To this end, we investigate swarms of the myxobacterium, Myxococcus xanthus. Individual M. xanthus cells are elongated; they always move in the direction of their long axis; and they are in constant motion, repeatedly touching each other. Remarkably, they regularly reverse their gliding directions. We have constructed a detailed cell-and behavior-based computational model of M. xanthus swarming that allows the organization of cells to be computed. By using the model, we are able to show that reversals of gliding direction are essential for swarming and that reversals increase the outflow of cells across the edge of the swarm. Cells at the swarm edge gain maximum exposure to nutrient and oxygen. We also find that the reversal period predicted to maximize the outflow of cells is the same (within the errors of measurement) as the period observed in experiments with normal M. xanthus cells. This coincidence suggests that the circuit regulating reversals evolved to its current sensitivity under selection for growth achieved by swarming. Finally, we observe that, with time, reversals increase the cell alignment, and generate clusters of parallel cells.gliding motility ͉ stochastic model ͉ pattern formation ͉ cell alignment ͉ oscillate

Myxococcus xanthus develops species-specific multicellular fruiting bodies. Starting from a uniform mat of cells, some cells enter into nascent fruiting body aggregates, whereas other cells remain outside. The cells within the fruiting body differentiate from rods into spherical, heat-resistant spores, whereas the cells outside the aggregates, called peripheral cells, remain rod-shaped. Early developmentally regulated genes are expressed in peripheral cells as well as by cells in the fruiting bodies. By contrast, late developmental genes are only expressed by cells within the nascent fruiting bodies. The data show that peripheral cells begin to develop, but are unable to express genes that are switched on later than about 6 h after the start of development. All of the genes whose expression is limited to the fruiting body are dependent on C-signaling either directly or indirectly, whereas the genes that are equally expressed in peripheral rods and in fruiting body cells are not. One of the C-signal-dependent and spatially patterned operons is called dev, and the dev operon has been implicated in the process of sporulation. It is proposed that expression of certain genes, including those of the dev operon, is limited to the nascent fruiting body because fruiting body cells engage in a high level of C-signaling. Peripheral cells do less C-signaling than fruiting body cells, because they have a different spatial arrangement and are at lower density. As a consequence, peripheral cells fail to express the late genes necessary for spore differentiation.spatial pattern ͉ positive feedback ͉ Myxobacteria ͉ cell-cell interaction H ow spatial patterns of differentiated cells arise is a central issue for animal and plant development. Myxococcus xanthus and other myxobacteria differentiate spores in response to nutrient deprivation. Although most bacteria sporulate individually, myxobacteria build large structured masses of spores, called fruiting bodies. Under nutrient-rich conditions, M. xanthus grows and divides as rod-shaped cells. When its development is induced by starvation, a hundred thousand cells contribute to building a fruiting body, whose shape is speciesspecific. Cells that have entered into the fruiting body finally differentiate into environmentally resistant myxospores, which can survive years without nutrients. However, not all of the starvation-induced cells become spores. Cells within fruiting bodies become spores, cells outside and between these multicellular structures remain rod-shaped and nonresistant (1). These cells, called peripheral rod cells, never become spores, despite their synthesis of two sporulation proteins, Tps and C (1). Dworkin and Gibson (2) showed that every cell innately has the capacity to become a spore. A difference in the developmental fate of peripheral rods and of fruiting body cells constitutes a spatial pattern that needs to be explained.Most patterns involve cell-to-cell signaling, and sporulation depends on C-signaling. Ordinarily, each cell is simultaneously a transmitter...

No abstract

The X repressor is the only phage protein needed to maintain immunity (1). By binding two operators, Or and 01 in Fig. 1, this protein directly prevents the transcription of the two early phage operons (2, 3). Consequently, only one operon, that which includes cI the structural gene for repressor (2), is transcribed in the presence of active repressor.Since Eisen et al. (4) (23,24). cro reduces N expression (25) and ci expression.Mutations used in this paper include: V3, vl, and vS3u, which map in Or (2, 11); sex, possibly in Pl, which reduces leftward transcription of N and cIII (26), and X3-and x13-, possibly in Pr, which abolish rightward transcription of cro, cdI, and 0 (27). The DNA deleted in two prophage strains is denoted below the genetic map. 2185