Luteolin, a flavone present in seed exudates of alfalfa, induces nodulation genes (nod) in Rhizobium meliloti and also serves as a biochemically specific chemoattractant for the bacterium. The present work shows that R. meliloti RCR2011 is capable of very similar chemotactic responses towards 4',7-dihydroxyflavone, 4',7-Dihydroxyflavanone, and 4,4'-dihydroxy-2-methoxychalcone, the three principal nod gene inducers secreted by alfalfa roots. Chemotactic responses to the root-secreted nod inducers in capillary assays were usually two-to four-fold above background and, for the flavone and flavonone, occurred at concentrations lower than those required for half-maximal induction of the nodABC genes. Complementation experiments indicated that the lack of chemotactic responsiveness to luteolin seen in nodDI and nodA mutants of R. meliloti was not due to mutations in the nod genes, as previously thought. Thus, while nod gene induction and flavonoid chemotaxis have the same biochemical specificity, these two functions appear to have independent receptors or transduction pathways. The wild-type strain was found to suffer selective, spontaneous loss of chemotaxis towards flavonoids during laboratory subculture.
Streptomyces has been known to form two types of septa. The data in this research demonstrated that Streptomyces griseus forms another type of septum near the base of sporogenic hyphae (basal septum). To understand the regulation of the septation machinery in S. griseus, we investigated the expression of the ftsZ gene. S1 nuclease protection assays revealed that four ftsZ transcripts were differentially expressed during morphological differentiation. The vegetative transcript (emanating from P veg ) is present at a moderate level during vegetative growth, but is switched off within the first 2 h of sporulation. Two sporulation-specific transcripts predominantly accumulated, and the levels increased by approximately fivefold together shortly before sporulation septa begin to form. Consistently, the sporulation-specific transcripts were expressed much earlier and more abundantly in a group of nonsporulating mutants that form their sporulation septa prematurely. Promoter-probe studies with two different reporter systems confirmed the activities of the putative promoters identified from the 5 end point of the transcripts. The levels and expression timing of promoter activities were consistent with the results of nuclease protection assays. The aseptate phenotype of the P spo mutant indicated that the increased transcription from P spo is required for sporulation septation, but not for vegetative or basal septum formation.The early discernible event in bacterial cell division is the involvement of FtsZ, which governs the localization of septum formation. ftsZ is present in all bacteria so far examined, including wall-less bacteria (55), as well as in archaea (34, 56) and plastids of Arabidopsis (42). FtsZ forms the leading edge of the cell division septum by migrating from random cytoplasmic locations to the internal face of the cell membrane at the division plane, where FtsZ polymerizes into a ring structure (7). In Escherichia coli, the Min proteins appear to permit this ring structure to form at the center of the cell rather than at either pole (12). In Bacillus subtilis, ftsZ is required for both binary fission and asymmetric septation (6). Sharing some structural and biochemical properties with the eucaryotic cytoskeletal protein tubulin, FtsZ possesses GTPase activity (11,40,46). Interaction of FtsZ with GTP appears to be necessary for polymerization in vitro (9, 41). The inward growth of the division septum is presumably accompanied by the controlled depolymerization of the FtsZ ring, the appositional sliding of polymeric subunits (9), or a profound change in the ring architecture.Although the role of FtsZ appears to be similar in procaryotes, the pattern of synthesis of FtsZ differs, apparently in compliance with the physiological demands of each species. The level of FtsZ from Caulobacter crescentus varies in concert with the division cycle; FtsZ reaches a maximal level in predivisional cells and is absent from swarmer cells, in which FtsZ appears to be specifically degraded (45). In E. coli, FtsZ is present...
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