Methylation involved in chemotaxis is regulated during Caulobacter differentiation.

Self Cite

Each Caulobacter crescentus cell division generates distinct progeny cells: a stalked cell which is competent for DNA replication, and a motile swarmer cell which initiates DNA replication only after it sheds its flagellum and differentiates into a stalked cell later in the cell cycle. Following the initiation of DNA replication, the flagellar transcriptional hierarchy is activated, culminating in the assembly of a single flagellum at the cell pole opposite that bearing the stalk. The biogenesis of the flagellum is a landmark in the establishment of asymmetry in the predivisional cell (Fig. 1).A large number of the flagellar structural and regulatory genes have been grouped into classes (Fig. 1) that form a regulatory hierarchy (7,9,11,43,62). We have preliminary evidence that the class I genes respond to cell cycle cues (44a). The class II flagellar genes are transcribed early in the cell cycle and are required for the transcription of genes in classes III and IV. Two class II genes, rpoN and flbD, encode a sigma factor ( 54 ) (6), and a transcriptional activator, FlbD (45, 60), respectively, which are required directly for the transcription of class III and class IV genes. Other class II genes encode proteins required for flagellar assembly and function (24, 45a, 65). The order of flagellar gene expression approximates the order of assembly of the gene products into the nascent structure (11,13,17,43,62). In C. crescentus, Salmonella typhimurium, and Escherichia coli, if flagellar assembly is aborted, transcription of the remaining flagellar genes is blocked, suggesting that the two processes are coupled (19,23,24,31).We reported previously that a nonmotile mutant with a deletion in the flaS locus not only diminished or abolished the transcription of class III and class IV genes but also exhibited a cell division defect (13). To understand the role of this class II flaS locus in the assembly of the flagellum and in cell divi- At each division, a predivisional cell gives rise to two different progeny, a stalked cell and a motile swarmer cell. As the cell cycle proceeds, the swarmer cell sheds its flagellum and assembles a stalk at the previously flagellated pole. The new stalked cell begins to divide, assembling a new flagellum at the pole opposite the stalk. The flagellar genes are expressed in an order that reflects their positions in the regulatory hierarchy and the order of assembly of their protein products into the flagellum (7,42). Arrows indicate positive regulation, such that transcription of each class of genes requires the expression of the gene products of the preceding class. Both rpoN and flbD encode transcription proteins (6, 45), whereas other class II genes appear to encode proteins involved in the assembly, structure, and function of the flagellum (24, 65).

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

Caulobacter FliQ and FliR membrane proteins, required for flagellar biogenesis and cell division, belong to a family of virulence factor export proteins

Zhuang

Shapiro

1995

Self Cite

“…This methylation-dependent pathway allows a very sensitive response to a range of different chemical stimuli. In almost all bacteria studied so far, MCPs have been identified by the use of antibodies raised against an E. coli MCP (1, 43) and by in vivo methylation assays (14,19,24,40,41,46). Only one bacterium, Rhodobacter sphaeroides, has been shown to lack MCPs (43) and to possess metabolism-dependent chemotaxis (36; for a review, see reference 4).…”

mentioning

confidence: 99%

Motility, chemokinesis, and methylation-independent chemotaxis in Azospirillum brasilense

Zhulin

Armitage

1993

Observations of free-swimming and antibody-tethered Azospirillum brasilense cells showed that their polar flagella could rotate in both clockwise and counterclockwise directions. Rotation in a counterclockwise direction caused forward movement of free-swimming cells, whereas the occasional change in the direction of rotation to clockwise caused a brief reversal in swimming direction. The addition of a metabolizable chemoattractant, e.g., malate or proline, had two distinct effects on the swimming behavior of the bacteria: (i) a short-term decrease in reversal frequency from 0.33 to 0.17 s5l and (ii) a long-term increase in the mean population swimming speed from 13 to 23 pm s'1. A. brasilense therefore shows both chemotaxis and chemokinesis in response to temporal gradients of some chemoeffectors. Chemokinesis was dependent on the growth state of the cells and may depend on an increase in the electrochemical proton gradient above a saturation threshold. Analysis of behavior of a methionine auxotroph, assays of in vivo methylation, and the use of specific antibodies raised against the sensory transducer protein Tar of Escherichia coli all failed to demonstrate the methylation-dependent pathway for chemotaxis in A. brasilense. The range of chemicals to which A. brasilense shows chemotaxis and the lack of true repellents indicate an alternative chemosensory pathway probably based on metabolism of chemoeffectors.Azospirillum brasilense is a soil bacterium living in close association with plant roots. It stimulates plant growth, probably because of its ability to fix nitrogen and to produce phytohormones (33). A. brasilense is capable of aerotaxis (8) and chemotaxis towards different organic compounds (7, 38, 49). Although there have been some studies of the chemosensory pathways in this organism (17, 37), very little is known about its mechanism of motility and chemotaxis. A. brasilense has a mixed pattern of flagellation; a single polar flagellum is synthesized during growth in liquid media, whereas lateral flagella are synthesized in addition to the polar flagellum during growth on solid media. The polar and lateral flagella are structurally and immunologically different, and they perform different motility functions. The polar flagellum is responsible for swimming motility, and the lateral flagella are responsible for swarming (18). In natural environments, migration ofA. brasilense towards plant roots is limited by soil moisture (9), suggesting that free swimming through the water space rather than swarming plays the major role in chemotactic behavior in natural environments.Oxygen and other electron acceptors are recognized byA. brasilense as chemoeffectors via a sensory pathway based on the functioning redox chain (17, 37). The aerotactic responses in Escherichia coli and Salmonella typhimurium also operate as a result of changes in electron flow through the redox chain and/or consequent changes in proton motive force (41, 42). It is possible that some chemicals which are also effective electron donors, such a...

“…Caulobacter crescentus is a gram-negative, dimorphic bacterium that has been used to study temporal gene expression (4,12,15,16,(20)(21)(22). Differentiation of C. crescentus is a developmentally programmed sequence of events that occurs during normal exponential growth and is not a response to nutritional deprivation as in other bacteria (17,20).…”

mentioning

confidence: 99%

Regulation of tryptophan biosynthesis in Caulobacter crescentus

Ross

Winkler

1988

We present an analysis of the expression of the trpE gene and the trpFBA operon in the dimorphic bacterium Caulobacter crescentus. The catalytic activity of component I of anthranilate synthase, the product of the trpE gene, was efficiently inhibited by tryptophan, the end product of the pathway, which suggests that tryptophan biosynthesis is likely controlled at the pathway level in C. crescentus. However, trpFBA mRNA levels and trpE enzyme levels did not vary significantly in wild-type C. crescentus in response to the presence of tryptophan in the growth medium or to growth in minimal versus rich medium. This lack of regulation of the trpE, trpF, trpB, and trpA genes is consistent with the idea that oligotrophic bacteria, such as C. crescentus, do not utilize regulatory mechanisms that greatly alter the biosynthetic capabilities in exponentially growing cells. In contrast, mRNA levels from the 5'-untranslated region and the upstream gene (usg) coding region increased dramatically in C. crescentus trpD or hisB auxotrophs starved for tryptophan or histidine, respectively. Surprisingly, concomitant increases in mRNA levels were not detected from the trpF, trpB, or trpA coding regions downstream in the operon. Thus, severe starvation of C. crescentus for amino acids appears to elicit a strong, general transcriptional response that is not observed in bacteria growing exponentially in medium lacking amino acids.