Figure 1. Cell-cycle localization of regulatory proteins involved in C. crescentus cell division and polar development. (a) Dimorphic life cycle of C. crescentus. Swarmer cell sheds its polar flagellum and retracts its pili prior to the swarmer to stalked (SW–ST) cell transition. A polar stalk is formed at the site of the ejected flagellum. Cell begins to elongate prior to cell division as an early predivisional cell. As a late predivision cell, a flagellum is formed and activated at the pole opposite the stalk. At cell separation a new swarmer cell is released and the stalked cell begins a new round of cell division. Localization of master cell-cycle regulator, (b) CtrA, histidine kinases (c) CckA and (d) DivJ, phosphotransfer protein, (e) DivK, histidine kinase, (f) PleC, and novel polar regulatory protein, (g) PodJ in the different stages of the Caulobacter life cycle. The short form of PodJ (PodJS) is indicated by filled circles, and the long form of PodJ (PodJL) is shown by concentric circles.

Figure 2. Sporulation of Bacillus subtilis. During vegetative growth (a) B. subtilis replicates its chromosome producing two copies of its genome. Each copy of the chromosome is segregated to an opposite pole of the cell. A division septum is placed medially and the ensuing cell division results in two essentially identical cells. During sporulation (b), the chromosomes remain together and are not segregated to opposite poles. Instead, the DNA forms an elongated structure called an axial filament which covers the entire length of a cell. The division septum is placed asymmetrically, close to one of the poles of the cell, forming a forespore compartment. This septum bisects the chromosome, but the rest of the DNA is transported to the interior of the forespore compartment. The forespore compartment is then fully engulfed by the mother cell. A spore coat is synthesized to protect the endospore, which is released following the lysis of the mother cell. From Levin and Grossman (1998).

Figure 3. B. subtilis sporulation phosphorelay. Phosphorelay begins with phosphorylation of SpoOF by KinA, KinB, KinC, KinD or KinE. Phosphorylation state of SpoOF can be changed by Rap phosphatases. SpoOF~P (phosphate) phosphorylates SpoOB. SpoOB~P passes a phosphate to SpoA. SpoOA~P activates polar cell division and sporulation. The default state for vegetatively growing cells is medial cell division.

Figure 4. Life cycle of M. xanthus. Scanning electron micrographs of (a) a lone cell under vegetative growth conditions, (b) a myxospore, (c) cells migration to a fruiting body and (d) a mature fruiting body structure. Bars: (A, B), 1 m; (C, D), 20 m. From Shimkets (1999).

Figure 5. Regulation of S. coelicolor development. In response to an unknown signal within the growing substrate mycelium, BldG activates transcription of the sigma factor, BldN. If BldD-mediated repression of BldN transcription is released, the accessory sigma factor will direct transcription of the response regulator BldM. BldM promotes aerial growth. WhiG, a sigma factor present in aerial hyphae activates transcription of WhiH and WhiI. WhiH and WhiI autoregulate their own promoters until such time, it is surmised, growth stops. WhiA and WhiB might play a role in sensing when growth slows. WhiH and WhiI then activate transcription of genes involved in sporulation septation. From Chater (2000).

Figure 6. Phase-contrast micrographs of filaments of Nostoc punctiforme. (a) Vegetative filaments grown in media containing ammonium. Heterocysts are not visible in any of the filaments. (b) Filaments grown in the absence of nitrogen in the culture medium. Heterocysts were observed with regular spacing as shown by the arrows. (c) Epifluorescence image of the same filaments from panel B (excitation 510–560nm emission >600 nm). The photosynthetic pigment phycoerythrin is present in vegetatively growing cells, but heterocysts do not possess the photosynthetic apparatus and do not fluoresce. From Meeks and Elhai (2002).