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Following C-signal transmission, step (1) in Figure 10 causes the expression of csgA to rise; this is the positive feedback that increases the intensity of signaling once it has started. In step
(2) of Figure 10, C-signaling activates FruA protein by a post-translational modification that is evident by its electrophoretic mobility (Ellehauge et al., 1998). The protein FruA is a DNA-bindingresponse regulator with a helixturn- helix (HTH) motif (Ellehauge et al., 1998; Ogawa et al., 1996). Activated FruA is necessary for aggregation and sporulation; there is no evidence that csgA expression is FruA-dependent. Synthesis of FruA protein depends on A-factor and E-factor, but not on C-factor (Figure 10).

The frz target of FruA is a phosphorelay, Figure 10 step (3), which modulates thefrequency of reversal of direction (Blackhart and Zusman, 1985). In that relay, the FrzCD protein has a domain that resembles the carboxy-end of a methyl-accepting chemotaxis protein (McBride et al., 1989). However, FrzCD protein is not a membrane receptor; it is found in the cytoplasm and has no transmembrane or extracellular domain. Activated FruA sends a signal along the frz phosphorelay, asdetected by the methylation of the FrzCD protein (Søgaard-Anderson and Kaiser, 1996a), and by a C-factor-induced increase in gliding speed and duration of gliding interval (Jelsbak and Søgaard-Andersen, 1999).

The methylation of FrzCD shifts during fruiting body development (Søgaard-Anderson and Kaiser, 1996a). Starting from a fully nonmethylated state early in development, the FrzCD proteingradually shifts to methylated states, and by 9 hours, the time of symmetrical mound building, all the FrzCD protein is methylated. Extracellular addition of purified C-factor to C-factor-less mutant cells directly and specifically induces the full methylation of their FrzCD protein, paralleling wild-type development (Søgaard- Anderson and Kaiser, 1996a). The cellular mechanism of aggregation, however,has not yet been established.

Both fruA and frz mutants were found in a screen of Tn5 insertion strains that arrested development in a state of partial aggregation, at the same stage as csgA mutants (Søgaard- Anderson et al., 1996b). Unlike csgA mutants, however, the targeted mutants are cellautonomous; they are not rescued by addition of wild-type cells, or by addition of purified Cfactor.Comparison of the properties of the fruA and frz mutants immediately showed that FruA is needed for aggregation and sporulation, but frz is only needed for aggregation.

The second target of activated FruA is the dev operon, Figure 10 step (4). Operon expression, as measured by the extent and time course of - galactosidase expression from a Tn5 Lac transcriptional fusion to devR, depends on FruA inthe same manner as it depends on C-factor (Ellehauge et al., 1998). The operon, in turn, is necessary for fruiting body sporulation; null mutants sporulate at 0.1% or less the frequency of wild type cells (Thony-Meyer and Kaiser, 1993). The morphological differentiation of myxospores occurs after aggregation is complete. In Sigmatella aurantiaca, another myxobacterium whose fruiting bodies have amore complex morphology (a stalk, branches and multiple cysts containing the spores), the aggregates pass through different intermediate shapes, including a myxococcus- like mound. Nevertheless myxospores do not form until the fruiting bodies are assuming their final branched shape (Qualls et al., 1978).

Activated FruA elevates the transcription of the dev operon. This operon has a switch-likequality, evident in the bimodality of operon expression in populations of developing cells (Russo-Marie et al., 1993). The two states may be consequences of the intense C-signaling in the densely packed cells that circulate inside nascent fruiting body aggregates and of little or no Csignaling in cells less densely arrayed in the periphery. Spores are found differentiated inside the fruiting...
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