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Azospirillum and R. centenaria species are also both capable of efficiently fixing nitrogen under aerobic growth conditions which has important agricultural implications (Steenhoudt and Vanderleyden 2000; Yildiz 1990). Genetic analysis of the R. centenaria life cycle is revealing a complex regulatory network that controls cellular differentiation processes (Berleman et al. 2004). A similar sensory transduction network for cyst differentiation is found in Azospirillum and in Azotobacter species. R. centenaria is thus a model organism for cyst cellular differentiation in proteobacteria (Jiang et al. 1998). R. centenaria and Azospirillum sp. both undergo similar cellular differentiation events involving swarm and cyst cells as shown in Figure 3.
When grown in liquid medium, R. centenaria swim cells are vibroid shaped containing a single polar flagellum. However, when grown on agar-solidified medium, the cells differentiate into rod-shaped swarm cells that are hyper-flagellated with lateral and polar flagella (Favinger et al. 1989; Stadtwald-Demchick et al. 1990; Nickens et al. 1996). Mutational analysis indicates that there is a complete duplication of flagella genes in R. centenaria with one set used for constitutive polar flagellum synthesis and the other set for surface induced lateral flagellum synthesis. Genetic analysis of swarm cell differentiation indicates that R. centenaria has a distantly different mechanism of inducing lateral flagellum synthesis over that of polar flagellum synthesis (Jiang et al. 1998; McClain et al. 2002). Synthesis of the polar flagellum is controlled by regulatory proteins similar to that used for controlling flagellum synthesis by Caulobacter sp. In contrast, surface induced synthesis of the lateral flagellum is controlled by several membrane spanning “receptors” that control activity of an alternative sigma factor.
One interesting feature of R. centenaria swarm cells is the ability of swarm colonies to rapidly phototaxis toward and away from light (a process that was featured in Nature and on the cover of J. Bacteriology) (Nickens et al. 1996; Ragatz et al. 1995; Jiang et al. 1997; Jiang and Bauer 2001). This unique characteristic of R. centenaria cells has allowed the first genetic and molecular genetic dissection of the process of phototaxis in a eubacterial cell.
In addition to swarm cell differentiation, when R. centenaria cells are challenged with nutrient limiting growth conditions, they undergo an additional differentiation process to form heat and desiccation resistant cysts (Fig. 4) (Favinger et al. 1989; Stadtwald-Demchick et al. 1990; Nickens et al. 1996; Berleman and Bauer 2003). Like endospores of Gram- positive bacteria, these cysts are a resting phase that allows survival under conditions of extreme temperature, drying or UV irradiation (Stadtwald-Demchick et al. 1990; Berleman and Bauer 2003). However, unlike endospores, cysts from R. centenaria, Azosprillum and Azotobacter cysts are not resistant to extremes in temperature (such as boiling). Instead the cysts are resistant to temperatures up to 65°C and are also very resistant to desiccation and exposure to UV irradiation (Stadtwald-Demchick et al. 1990; Berleman and Bauer 2003). At this time, there is little in the literature (at a genetic and molecular genetic level) about how any of these species makes cysts. However, the Bauer laboratory has been undertaking extensive genetic analysis of cellular differentiation which has led to the isolation of numerous Tn5 tagged mutants that are defective in different stages of cyst differentiation (Berleman et al. 2004). Numerous regulatory genes (three sensor kinases and two response regulators) have been identified that regulate induction of cyst differentiation (Berleman et al. 2004). Several of the regulatory proteins are conserved in both Azospirillum and in R. centenaria so it seems likely that ongoing studies of cyst formation in R. centenaria will have significance for a number of cyst forming species.
One of the more surprising features of R. centenaria is that existence of plant like genes in its genome. Specifically, R. centenaria contains a unique phytochrome photoreceptor that controls gene expression in response to blue and red light (Jiang et al. 1999). Until recently, phytochromes were considered to be plant specific photoreceptors that control plant development (shoot elongation and the timing of flowering etc.) in response to red light. Recent genome sequencing studies in other species have confirmed that plant phytochromes are actually of bacterial origin (Hughes and Lamparter 1999).
The R. centenaria genome is 4.15 Mb as based on pulse field electrophoretic analysis (unpublished). The genome GC content is 68.3% G+C which is typical of an alpha-1 proteobacteria. |
