Experimental evolution of catabolic pathways of bacteria

Experimental evolution of catabolic pathways offers considerable potential for accelerating the evolution of bacteria able to degrade toxic industrial chemicals, and this may be useful for combatting environmental pollution. The principal strategies that have been successfully followed to evolve useful catabolic routes for aromatic compounds in soil bacteria are analysed.

The xylS gene positive regulator of TOL plasmid pWWO: identification, sequence analysis and overproduction leading to constitutive expression of meta cleavage operon

The Pseudomonas putida TOL plasmid pWWO carries an operon that specifies a meta-cleavage pathway for the catabolism of benzoate and toluates whose transcription is positively regulated by the xylS gene product. Stimulation of transcription of the operon is thought to result from activation of this protein by pathway substrates/effectors. In the present study, overexpression of the xylS gene has led to identification of the regulator as a 33 kDa protein. Overexpression of xylS also resulted in partially constitutive, i.e. effector-independent expression of the meta-cleavage operon. Determination of the polynucleotide sequence of the xylS gene revealed amino acid sequence homology with several DNA binding proteins, particularly with the araC products of Escherichia coli and Salmonella typhimurium and with the nifA and ntrC products of Klebsiella pneumoniae. Homologous sequences were mainly located in an alpha-helix-turn-alpha-helix domain of the polypeptide. Interestingly, amino acid sequence homology was also found with sigma factors of E. coli (ntrA and htpR products) and Bacillus subtilis (spoIIG and phage SPOI Gp34 products) and other RNA polymerase core-interacting proteins, such as the E. coli nusA product.

Redesigning metabolic routes: manipulation of TOL plasmid pathway for catabolism of alkylbenzoates

Increasing quantities of man-made organic chemicals are released each year into the biosphere. Some of these compounds are both toxic and relatively resistant to physical, chemical, or biological degradation, and they thus constitute an environmental burden of considerable magnitude. Genetic manipulation of microbial catabolic pathways offers a powerful means by which to accelerate evolution of biodegradative routes through which such compounds might be eliminated from the environment. In the experiments described here, a catabolic pathway for alkylbenzoates specified by the TOL plasmid of Pseudomonas was restructured to produce a pathway capable of processing a new substrate, 4-ethylbenzoate. Analysis of critical steps in the TOL pathway that prevent metabolism of 4-ethylbenzoate revealed that this compound fails to induce synthesis of the catabolic enzymes and that one of its metabolic intermediates inactivates catechol 2,3-dioxygenase (C23O), the enzyme that cleaves the aromatic ring. Consequently, the pathway was sequentially modified by recruitment of genes from mutant bacteria selected for their production of either an altered pathway operon regulator that is activated by 4-ethylbenzoate or an altered C23O that is less sensitive to metabolite inactivation. The redesigned pathway was stably expressed and enabled host bacteria to degrade 4-ethylbenzoate in addition to the normal substrates of the TOL pathway.

Altered effector specificities in regulators of gene expression: TOL plasmid xylS mutants and their use to engineer expansion of the range of aromatics degraded by bacteria

Stimulation of transcription from positively regulated promoters involves regulatory proteins that have been activated, generally as a consequence of binding low molecular weight effector molecules. To define essential structural features of effectors for one positively acting gene regulator, the xylS-encoded protein, which activates the TOL plasmid meta-cleavage pathway operon promoters, effector activities of a wide range of benzoate derivatives have been systematically analyzed, and mutant xylS-encoded proteins exhibiting altered effector specificities have been generated and characterized. Cloned mutant xylS genes were trans dominant in partial diploids containing the wild-type xylS allele and could therefore be used to effect expansion of the range of aromatic compounds completely or partially degraded by Pseudomonas bacteria. The method developed to isolate mutant xylS-encoded proteins has general applicability and could in principle be used to isolate gene regulator specificity mutants of any inducible regulatory system.

Vector for regulated expression of cloned genes in a wide range of gram-negative bacteria

A pKT231-based broad-host-range plasmid vector was constructed which enabled regulation of expression of cloned genes in a wide range of gram-negative bacteria. This vector, pNM185, contained upstream of its EcoRI, SstI, and SstII cloning sites the positively activated pm twin promoters of the TOL plasmid and xylS, the gene of the positive regulator of these promoters. Expression of cloned genes was induced with micromolar quantities of benzoate or m-toluate, the inexpensive coinducers of the pm promoters. Expression of a test gene, xylE, which specifies catechol 2,3-dioxygenase, cloned in this vector was tested in representative strains of a variety of gram-negative bacteria. Regulated expression of xylE was observed in most strains examined, and induced levels of enzyme representing up to 5% of total cellular protein and ratios of induced:noninduced levels of enzyme up to a factor of 600 were observed. The level of xylE gene expression in different bacteria tended to be correlated with their phylogenetic distance from Pseudomonas putida.

[Syndrome of calcifications of basal ganglia, leukodystrophy and chronic lymphomonocytic pleocytosis of the cerebrospinal fluid: report of a case]

Norman & Tingey (1966) reported a new syndrome of micrencephaly, strio-cerebellar calcifications and leucodystrophy and in 1968, Lyon & col. reported the same syndrome plus dwarfism. These authors did not describe cerebrospinal fluid (CSF) alterations. In 1984, Aicardi & Goutièrres described 8 children from 5 different families with a syndrome like above referred to but with chronic CSF lymphocytosis; all patients had a progressive evolution, with familial character, with probable autosomic recessive heritage. It is the purpose of this report to relate a case similar to Aicardi & Goutièrres' cases in a male caucasian patient of jewish ashkenazim origin with 6 and a half month of age, but with a study of CSF lymphocytes T and B. Diminished T-subpopulations of active and avid-T were found suggesting local signalization of antibodies in the central nervous system. The differential diagnosis and that strange combination of a probable genetic etiology and an immunitary process revealing a local inflammatory process are discussed.

Subcutaneous mucormycosis caused by Rhizopus oryzae probable nosocomial acquired infection

The Authors present a case of subcutaneous mucormycosis occurring in a patient with clinical and biochemical evidence of diabetic ketoacidosis. The clinical, mycological and histopathological features are described, emphasizing the relevance of a rapid diagnosis in order to stablish early treatment. The clinical forms of mucormycosis and the main associated conditions are briefly reviewed as well as the most probable conditions which may lead to the enhanced susceptibility to infection in the diabetic patient in ketoacidosis. The recovery of Rhizopus oryzae from the air of the room of the patient suggests a nosocomial infection acquired through contamination of venous puncture site by air borne spores.

Lesions in citrate synthase that affect aerobic nitrogen fixation by Azotobacter chroococcum

A class of Azotobacter chroococcum mutants induced by Tn1 that were defective in normal aerobic nitrogen fixation when grown on sugars (Fos-) were corrected by provision of alpha-ketoglutarate or glutamate. In a representative mutant, Fos252, rates of evolution of 14CO2 from [14C]acetate or [14C]glucose were 5% of the parental values, although uptake and incorporation were normal for both substrates. The results suggest that a lesion affects the entry of substrates into the tricarboxylic acid cycle. The activity of citrate synthase in Fos252 in vitro was 5% that of the parents. The citrate synthase (gltA) gene from Escherichia coli was cloned into broad-host-range vectors and mobilized into Fos252. The plasmids restored parental citrate synthase activities to Fos252 and complemented the inability to fix N2 in air. The data indicate that a mutation causing an intrinsic limitation in respiratory capacity abolishes normal aerobic N2 fixation, which is consistent with the hypothesis of respiratory protection for nitrogenase in Azotobacter species.

Optimization of Conditions for Photoproduction of Ammonia from Nitrate by Anacystis nidulans

The effect of several relevant environmental factors influencing the photoproduction of ammonia from nitrate by Anacystis nidulans cells treated with the glutamine synthetase inhibitor l -methionine- dl -sulfoximine has been investigated. The optimal ratio between l -methionine- dl -sulfoximine concentration (micro-molar) and cell density (micrograms of chlorophyll per milliliter) was around 1, the process taking place at maximal rate at a temperature of about 40°C, within the pH range of 7 to 10. Ammonia production was stimulated by CO 2 or bicarbonate and was not affected by the accumulation of ammonia in the medium up to concentrations of 30 mM. The rate of ammonia production was found to be determined by the interaction of at least four factors, namely, irradiance and the density, depth, and turbulence of the cell suspension. Ammonia photoproduction from nitrate and water represents an interesting process for the conversion of light energy into chemical energy, which can operate at high efficiency, around 30% of its theoretical maximum.

Sustained Photoproduction of Ammonia from Nitrate by Anacystis nidulans

Conditions that lengthen the time during which l -methionine- dl -sulfoximine (MSX) promotes excretion of ammonia produced by photosynthetic nitrate reduction in Anacystis nidulans have been sought. If MSX was added every 24 h, maximal rates of ammonia production were maintained for 3 days. After this time, ammonia production ceased due to a specific deficiency of glutamine in the cells, which finally led to cell lysis. The effective ammonia production period could be further extended either by adding a low amount of glutamine at the end of the 3-day period or by allowing the cells to recover for 8 h in the absence of MSX after every 48-h period in the presence of inhibitor. In this way, a steady production of ammonia lasting for at least 10 days was achieved. The MSX-treated cyanobacterial cells thus represent a system relatively stable with time for the conversion of light energy into chemical energy through the photoreduction of nitrate to ammonia.