The image below shows filaments of the cyanobacterium Anabaena . The arrow points to a heterocyst. At least three programmed DNA rearrangements occur during heterocyst differentiation in Anabaena. The rearrangements involve the excision of DNA elements from the chromosome by site-specific recombination between short directly repeated sequences. Excision of two elements is shown below.
Brackets indicate chains of contiguous heterocysts.
The myxobacteria are an interesting family of gliding bacteria that produce fruiting bodies in starvation conditions. When starvation conditions prevail and cell densities are above a threshold level, the cells migrate back along the slime tracks, aggregating by chemotaxis, to form large mounds of cells. As the vegetative cells migrate upwards into the fruiting body they undergo a progressive differentiation into spherical, thick-walled spores. A mixture of spherical spores and rod-shaped vegetative cells is shown in panel D . A number of extracellular signals are necessary for fruiting body development, and mutants that have lost the ability to produce these extracellular signals have been isolated.