Search

Hydroxyproline (Hyp) in decaying organic matter is a rich source of carbon and nitrogen for microorganisms. A bacterial pathway for Hyp catabolism is known; however, genes and function relationships are not established. In the pathway, trans-4-hydroxy-L-proline (4-L-Hyp) is epimerized to cis-4-hydroxy-D-proline (4-D-Hyp), and then, in three enzymatic reactions, the D-isomer is converted via Δ-pyrroline-4-hydroxy-2-carboxylate (HPC) and α-ketoglutarate semialdehyde (KGSA) to α-ketoglutarate (KG). Here a transcriptional analysis of cells growing on 4-L-Hyp, and the regulation and functions of genes from a Hyp catabolism locus of the legume endosymbiont Sinorhizobium meliloti are reported. Fourteen hydroxyproline catabolism genes (hyp), in five transcripts hypR, hypD, hypH, hypST and hypMNPQO(RE)XYZ, were negatively regulated by hypR. hypRE was shown to encode 4-hydroxyproline 2-epimerase and a hypRE mutant grew with 4-D-Hyp but not 4-L-Hyp. hypO, hypD and hypH are predicted to encode 4-D-Hyp oxidase, HPC deaminase and α-KGSA dehydrogenase respectively. The functions for hypS, hypT, hypX, hypY and hypZ remain to be determined. The data suggest 4-Hyp is converted to the tricarboxylic acid cycle intermediate α-ketoglutarate via the pathway established biochemically for Pseudomonas. This report describes the first molecular characterization of a Hyp catabolism locus., (© 2012 Blackwell Publishing Ltd.)

TraR of Agrobacterium tumefaciens is a member of the LuxR family of transcriptional regulators, and binds to specific DNA sequences (tra boxes) at target promoters of the tumour-inducing (Ti) plasmid. Each tra box has a pronounced dyad symmetry, and each subunit of a TraR dimer binds to one half of a tra box via a helix-turn-helix (HTH) DNA binding motif. Structural analysis has suggested that TraR makes extensive sequence-specific contacts with tra box DNA. In this study, we tested these predictions using synthetic self-complementary oligonucleotides containing variant tra box sequences. Some predictions made from structural analysis were confirmed, while others were shown to be incorrect. Unexpectedly, these experiments also showed that six nucleotides at the centre of the tra box that make no direct contact with TraR are nevertheless critical for high-affinity binding and probably act by facilitating a previously described DNA bend. Variant tra boxes were also tested for transcription activity in vivo. Most transcription assays reflected in vitro binding assays. However, alterations of the outermost nucleotides had little effect on TraR binding but blocked transcription, probably by altering an overlapping -35 promoter motif.

Burkholderia cenocepacia is an opportunistic human pathogen that can aggressively colonize the cystic fibrosis lung. This organism has a LuxR/LuxI-type quorum sensing system that enables cell-cell communication via exchange of acyl homoserine lactones (AHLs). The CepR and CepI proteins constitute a global regulatory system, controlling expression of at least 40 genes, including those controlling swarming motility and biofilm formation. In this study, we isolated seven lacZ fusions in a clinical isolate of B. cenocepacia that are inducible by octanoyl-HSL. Induction of all of these genes requires CepR. The cepI promoter was tested for induction by a set of 33 synthetic autoinducers and analogues, and was most strongly induced by long-chain AHLs lacking 3-oxo substitutions. Expression of this promoter was inhibited by high concentrations of three different autoinducers, each having six-carbon acyl chains. When CepR protein was overproduced in Escherichia coli, it accumulated in a soluble form in the presence of octanoyl-HSL, but accumulated only as insoluble inclusion bodies in its absence. Purified CepR-OHL complexes bound to specific DNA sequences at the cepI and aidA promoters with high specificity. These binding sites included a 16-nucleotide imperfect dyad symmetry. Both CepR binding sites are centred approximately 44 nucleotides upstream of the respective transcription start sites.

The LuxR-type quorum-sensing transcription factor TraR regulates replication and conjugal transfer of the tumour-inducing (Ti) plasmid in the plant pathogen Agrobacterium tumefaciens. TraR is a two-domain protein with an N-terminal domain that binds to the quorum-sensing signal N-3-oxooctanoyl- l-homoserine lactone (OOHL) and a C-terminal domain that binds to specific DNA sequences called tra boxes. TraR-OOHL complexes form homodimers that activate transcription of at least seven promoters on the Ti plasmid. At five promoters, a tra box overlaps the binding site of core RNA polymerase (class II promoters), while in the other two promoters, this site is located farther upstream (class I promoters). In this study, we performed saturating point mutagenesis of the surface residues of the TraR C-terminal domain. Each mutant was tested for proteolytic stability and transcription activity in vivo, and for DNA binding activity in vitro. Mutants of TraR with single substitutions at positions W184, V187, K189, E193Q, V197 and D217 have wild-type levels of accumulation and DNA binding, but are defective in transcription of both types of promoters. These residues constitute a patch on the surface of the DNA-binding domain. We propose that this patch is an activating region that recruits RNA polymerase to TraR-dependent promoters through direct contact. As residues of this patch are critical for activation at both a class I and a class II promoter, we predict that these residues may contact the C-terminal domain of the RNA polymerase alpha-subunit.