Pseudomonas putida metabolizes Phe and Tyr through a peripheral pathway involving hydroxylation of Phe to Tyr (PhhAB), conversion of Tyr into 4-hydroxyphenylpyruvate (TyrB), and formation of homogentisate (Hpd) as the central intermediate. Homogentisate is then catabolized by a central catabolic pathway that involves three enzymes, homogentisate dioxygenase (HmgA), fumarylacetoacetate hydrolase (HmgB), and maleylacetoacetate isomerase (HmgC), finally yielding fumarate and acetoacetate. Whereas the phh , tyr , and hpd genes are not linked in the P. putida genome, the hmgABC genes appear to form a single transcriptional unit. Gel retardation assays and lacZ translational fusion experiments have shown that hmgR encodes a specific repressor that controls the inducible expression of the divergently transcribed hmgABC catabolic genes, and homogentisate is the inducer molecule. Footprinting analysis revealed that HmgR protects a region in the Phmg promoter that spans a 17-bp palindromic motif and an external direct repetition from position −16 to position 29 with respect to the transcription start site. The HmgR protein is thus the first IclR-type regulator that acts as a repressor of an aromatic catabolic pathway. We engineered a broad-host-range mobilizable catabolic cassette harboring the hmgABC , hpd , and tyrB genes that allows heterologous bacteria to use Tyr as a unique carbon and energy source. Remarkably, we show here that the catabolism of 3-hydroxyphenylacetate in P. putida U funnels also into the homogentisate central pathway, revealing that the hmg cluster is a key catabolic trait for biodegradation of a small number of aromatic compounds.

The paa cluster of Escherichia coli W involved in the aerobic catabolism of phenylacetic acid (PA) has been cloned and sequenced. It was shown to map at min 31.0 of the chromosome at the right end of the mao region responsible for the transformation of 2-phenylethylamine into PA. The 14 paa genes are organized in three transcription units:paaZ and paaABCDEFGHIJK, encoding catabolic genes; and paaXY, containing thepaaX regulatory gene. The paaK gene codes for a phenylacetyl-CoA ligase that catalyzes the activation of PA to phenylacetyl-CoA (PA-CoA). The paaABCDE gene products, which may constitute a multicomponent oxygenase, are involved in PA-CoA hydroxylation. The PaaZ protein appears to catalyze the third enzymatic step, with the paaFGHIJ gene products, which show significant similarity to fatty acid β-oxidation enzymes, likely involved in further mineralization to Krebs cycle intermediates. Three promoters, Pz, Pa, and Px, driven the expression of genes paaZ, paaABCDEFGHIJK, and paaX, respectively, have been identified. ThePa promoter is negatively controlled by thepaaX gene product. As PA-CoA is the true inducer, PaaX becomes the first regulator of an aromatic catabolic pathway that responds to a CoA derivative. The aerobic catabolism of PA in E. coli represents a novel hybrid pathway that could be a widespread way of PA catabolism in bacteria.

Fourteen different genes included in a DNA fragment of 18 kb are involved in the aerobic degradation of phenylacetic acid by Pseudomonas putida U. This catabolic pathway appears to be organized in three contiguous operons that contain the following functional units: ( i ) a transport system, ( ii ) a phenylacetic acid activating enzyme, ( iii ) a ring-hydroxylation complex, ( iv ) a ring-opening protein, ( v ) a β-oxidation-like system, and ( vi ) two regulatory genes. This pathway constitutes the common part ( core ) of a complex functional unit ( catabolon ) integrated by several routes that catalyze the transformation of structurally related molecules into a common intermediate (phenylacetyl-CoA).

The term catabolon was introduced to define a complex functional unit integrated by different catabolic pathways, which are, or could be, co‐ordinately regulated, and that catalyses the transformation of structurally related compounds into a common catabolite. The phenylacetyl‐CoA catabolon encompasses all the routes involved in the transformation of styrene, 2‐phenylethylamine, trans‐styrylacetic acid, phenylacetaldehyde, phenylacetic acid, phenylacetyl amides, phenylacetyl esters and n‐phenylalkanoic acids containing an even number of carbon atoms, into phenylacetyl‐CoA. This common intermediate is subsequently catabolized through a route of convergence, the phenylacetyl‐CoA catabolon core, into general metabolites. The genetic organization of this central route, the biochemical significance of the whole functional unit and its broad biotechnological applications are discussed.

The interaction ofBorrelia burgdorferi, the causative agent of Lyme borreliosis, with phagocytic cells induces the activation of NF-κB and the expression of proinflammatory cytokines including tumor necrosis factor alpha (TNF-α).B. burgdorferi-induced TNF-α production is also dependent on the activation of p38 mitogen-activated protein (MAP) kinase. The specific contribution of these signaling pathways to the response of phagocytic cells to the spirochete and the molecular mechanisms underlying this response remain unresolved. We now show that p38 MAP kinase activity regulates the transcriptional activation of NF-κB in response to spirochetal lysate stimulation of phagocytic cells. The regulation occurs at the nuclear level and is independent of the translocation of the transcription factor to the nucleus or its capacity to bind to specific DNA target sequences. In RAW264.7 cells, p38α MAP kinase regulates the phosphorylation of NF-κB RelA. p38 MAP kinase phosphorylates the nuclear kinase mitogen- and stress-activated protein kinase 1 (MSK1). MSK1 in turn phosphorylates the transcriptionally active subunit of NF-κB, RelA. The repression of MSK1 expression with small interfering RNA results in reduced RelA phosphorylation and a significant decrease in the production of TNF-α in response toB. burgdorferilysates. Overall, these results clarify the contribution of the signaling pathways that are activated in response to the interaction of spirochetes with phagocytic cells to TNF-α production. Our results situate p38 MAP kinase activity as a central regulator of the phagocytic proinflammatory response through MSK1-mediated transcriptional activation of the transcription factor NF-κB.