Danielle O'Donnol

Regulation of stalk elongation by phosphate in Caulobacter crescentus.

In Caulobacter crescentus, stalk biosynthesis is regulated by cell cycle cues and by extracellular phosphate concentration. Phosphate-starved cells undergo dramatic stalk elongation to produce stalks as much as 30 times as long as those of cells growing in phosphate-rich medium. To identify genes involved in the control of stalk elongation, transposon mutants were isolated that exhibited a long-stalk phenotype irrespective of extracellular phosphate concentration. The disrupted genes were identified as homologues of the high-affinity phosphate transport genes pstSCAB of Escherichia coli. In E. coli, pst mutants have a constitutively expressed phosphate (Pho) regulon. To determine if stalk elongation is regulated by the Pho regulon, the Caulobacter phoB gene that encodes the transcriptional activator of the Pho regulon was cloned and mutated. While phoB was not required for stalk synthesis or for the cell cycle timing of stalk synthesis initiation, it was required for stalk elongation in response to phosphate starvation. Both pstS and phoB mutants were deficient in phosphate transport. When a phoB mutant was grown with limiting phosphate concentrations, stalks only increased in length by an average of 1.4-fold compared to the average 9-fold increase in stalk length of wild-type cells grown in the same medium. Thus, the phenotypes of phoB and pst mutants were opposite. phoB mutants were unable to elongate stalks during phosphate starvation, whereas pst mutants made long stalks in both high- and low-phosphate media. Analysis of double pst phoB mutants indicated that the long-stalk phenotype of pst mutants was dependent on phoB. In addition, analysis of a pstS-lacZ transcriptional fusion showed that pstS transcription is dependent on phoB. These results suggest that the signal transduction pathway that stimulates stalk elongation in response to phosphate starvation is mediated by the Pst proteins and the response regulator PhoB.

Mutations in DivL and CckA Rescue a divJ Null Mutant of Caulobacter crescentus by Reducing the Activity of CtrA

Polar development and cell division in Caulobacter crescentus are controlled and coordinated by multiple signal transduction proteins. divJ encodes a histidine kinase. A null mutation in divJ results in a reduced growth rate, cell filamentation, and mislocalized stalks. Suppressor analysis of divJ identified mutations in genes encoding the tyrosine kinase (divL) and the histidine kinase (cckA). The divL and cckA suppressor alleles all have single amino acid substitutions, some of which confer a temperature-sensitive phenotype, particularly in a wild-type background. Analysis of transcription levels from several positively regulated CtrA-dependent promoters reveals high expression in the divJ mutant, suggesting that DivJ normally serves to reduce CtrA activity. The divL and cckA suppressors reduce the amount of transcription from promoters positively regulated by CtrA, indicating that the mutations in divL and cckA are suppressing the defects of the divJ mutant by reducing the abnormally high level of CtrA activity. Immunoblotting showed no major perturbations in the CtrA protein level in any of these strains, suggesting that the high amount of CtrA activity seen in the divJ mutant and the reduced amount of activity in the suppressors are regulated at the level of activation and not transcription, translation, or degradation. In vivo phosphorylation assays confirmed that divJ mutants have elevated levels of CtrA phosphorylation and that this level is reduced in the suppressors with mutations in divL.

INTRINSICALLY n-LINKED COMPLETE BIPARTITE GRAPHS

We prove that every embedding of K2n+1,2n+1 into ℝ3 contains a non-split link of n components. Further, given an embedding of K2n+1,2n+1 in ℝ3, every edge of K2n+1,2n+1 is contained in a non-split n-component link in K2n+1,2n+1.

KNOTTING AND LINKING IN THE PETERSEN FAMILY

Abstract This paper extends the work of Nikkuni [4] finding an explicit relationship for the graph K3,3,1 between knotting and linking, which relates the sum of the squares of linking numbers of links in the embedding and the second coefficient of the Conway polynomial of certain cycles in the embedding. Then we use this and other similar relationships to better understand the relationship between knotting and linking in the Petersen family. The Petersen family is the set of minor minimal intrinsically linked graphs. We prove that if such a spatial graph is complexly algebraically linked then it is knotted.

Legendrian theta-graphs

In this article we give necessary and sufficient conditions for two triples of integers to be realized as the Thurston-Bennequin number and the rotation number of a Legendrian theta-graph with all cycles unknotted. We show that these invariants are not enough to determine the Legendrian class of a topologically planar theta-graph. We define the transverse push-off of a Legendrian graph and we determine its self linking number for Legendrian theta-graphs. In the case of topologically planar theta-graphs, we prove that the topological type of the transverse push-off is that of a pretzel link.

Intrinsic 3-linkedness is not preserved by Y∇ moves

This paper introduces a number of new intrinsically 3-linked graphs through five new constructions. We prove that intrinsic 3-linkedness is not preserved by Y∇ moves. We will see that the graph M, which is obtained through a Y∇ move on (PG)∗∗(PG), is not intrinsically 3-linked.