Biosynthesis and control of beta-lactam antibiotics: the early steps in the "classical" tripeptide pathway

The interaction between growth and secondary metabolism develops from physiological responses of the producer organism to its environment. Nutrients are channelled into primary growth processes or into secondary processes such as antibiotic biosynthesis by a variety of metabolic controls, the nature of which has been extensively studied in organisms producing beta-lactam antibiotics via the tripeptide, delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine. In the following article we review the early stages of beta-lactam biosynthesis in fungi and actinomycetes, keeping in mind the regulation of primary pathways that provide the amino acid precursors of this group of antibiotics, as well as the regulation of the secondary pathway itself. Of special importance to organisms engaging in secondary metabolism are the control mechanisms that suppress the nonessential process during rapid growth but allow secondary metabolic genes to be expressed and resources to be diverted when environmental factors generate the appropriate biochemical signals.     

Role of the carbon source in regulating chloramphenicol production by Streptomyces venezuelae: studies in batch and continuous cultures

Both carbon- and nitrogen-limited media that supported a biphasic pattern of growth and chloramphenicol biosynthesis were devised for batch cultures of Streptomyces venezuelae. Where onset of the idiophase was associated with nitrogen depletion, a sharp peak of arylamine synthetase activity coincided with the onset of antibiotic production. The specific activity of the enzyme was highest when the carbon source in the medium was also near depletion at the trophophase-idiophase boundary. In media providing a substantial excess of carbon source through the idiophase, the peak specific activity was reduced by 75%, although the timing of enzyme synthesis was unaltered. Moreover, chemostat cultures in which the growth rate was limited by the glucose concentration in the input medium failed to show a decrease in specific production of chloramphenicol as the steady-state intracellular glucose concentration was increased. The results suggest that a form of "carbon catabolite repression" regulates synthesis of chloramphenicol biosynthetic enzymes during a trophophase-idiophase transition induced by nitrogen starvation. However, this regulatory mechanism does not establish the timing of antibiotic biosynthesis and does not function during nitrogen-sufficient growth in the presence of excess glucose.     

Conjugational fertility and location of chloramphenicol biosynthesis genes on the chromosomal linkage map of Streptomyces venezuelae

In Streptomyces venezuelae fertility, defined as chromosomal gene recombination, was enhanced over 1000-fold when one parent in a biparental conjugational cross lacked the physically-undetected plasmid SVP1, as compared with crosses in which both parents carried SVP1. The existence of SVP1 and at least two other fertility plasmids, SVP2 and SVP3, was detected in S. venezuelae by 'lethal zygosis' elicited by a plasmid-plus mycelium in contact with a plasmid-minus mycelium. Conjugational crosses were used to construct a linkage map of S. venezuelae which was highly consistent with the map of analogous loci in S. coelicolor A3(2). A cluster of genes governing chloramphenicol biosynthesis was located near arg, cys and pdxB genes at a position roughly equivalent to the 1-2 o'clock region of the S. coelicolor A3(2) map.     

Purification, properties and immunological detection of a bromoperoxidase-catalase from Streptomyces venezuelae and from a chloramphenicol-nonproducing mutant

A new bromoperoxidase-catalase was purified from the chloramphenicol-producing actinomycete Streptomyces venezuelae ISP 5230. The homogeneous enzyme showed brominating activity, catalase activity and a very low peroxidase activity. The spectral properties and pH dependence of the catalase activity showed similarities to conventional catalases. In contrast to other haem-bromoperoxidases, the bromoperoxidase-catalase was stable when treated with an ethanol/chloroform mixture. Gel filtration gave an estimated Mr of 127,000-136,000. SDS-PAGE showed a single band corresponding in mobility to a species with an Mr of 61,000. The pI was estimated to be 4.5. The bromoperoxidase-catalase was not present in active form in a mutant of S. venezuelae ISP 5230, blocked in the chlorination step of chloramphenicol biosynthesis. However, an inactive species of the enzyme was detected in crude extracts of the mutant by using antibodies. From these results it is concluded that this bromoperoxidase participates in the chlorination step during chloramphenicol biosynthesis.     

Interactions ofCandida albicans with bacteria and salivary molecules in oral biofilms

The yeastCandida albicans coaggregates with a variety of streptococcal species, an interaction that may promote oral colonization by yeast cells.C. albicans andCandida tropicalis are the yeasts most frequently isolated from the human oral cavity and our data demonstrate that both these species bind toStreptococcus gordonii NCTC 7869 while two otherCandida species (Candida krusei andCandida kefyr) do not. Adherence ofC. albicans was greatest when the yeast had been grown at 30° C to mid-exponential growth phase. For 21 strains ofC. albicans there was a positive correlation between the ability to adhere toS. gordonii and adherence to experimental salivary pellicle. Whole saliva either stimulated or slightly inhibited adherence ofC. albicans toS. gordonii depending on the streptococcal growth conditions. The results suggest that the major salivary adhesins and coaggregation adhesins ofC. albicans are co-expressed.     

Use of 13C in biosynthetic studies. The labelling pattern in tenellin enriched from isotope-labelled acetate, methionine, and phenylalanine

The biogenetic origin of the carbon atoms in tenellin has been established by adding 13C-enriched compounds to cultures of Beauveria bassiana, and determining the isotopic distribution in the metabolite by 13C nuclear magnetic resonance spectrometry. Tenellin is formed by condensation of an acetate-derived polyketide chain with a phenylpropanoid unit that may be phenylalanine. Alternate carbon atoms of the polyketide chain were labelled with sodium [1(-13C)]- and [2-(13C]-acetate; sodium [1,2-(13C)]acetate was incorporated as intact two-carbon units, the presence of which in tenellin was apparent from coupling between adjacent 13C-enriched carbons. Substituent methyl groups of the polyketide-derived alkenyl chain were labelled with L-[Me-13C]methionine. The labelling patterns from DL-[carboxy-13C]phenylalanine and DL-[alpha-13C]phenylalanine indicated a rearrangement of the propanoid component at some stage in the synthesis. The mass spectrum of tenellin from cultures administered L-[15N]phenylalanine showed isotopic enrichment similar to that obtained with 13C- or 14C-labelled phenylalanine. During incorporation of L-[carboxy-14C, beta-3H]phenylalanine 96% of the tritium label was lost, discounting the possibility of a 1,2-hydride shift during biosynthesis of the metabolite.     

Catabolite repression in Streptomyces venezuelae. Induction of beta-galactosidase, chloramphenicol production, and intracellular cyclic adenosine 3',5'-monophosphate concentrations

Chloramphenicol production was studied in cultures of Streptomyces venezuelae growing in a simple buffered medium with ammonia as the nitrogen source and glucose, lactose, or a glucose-lactose mixture as the sole source of carbon. With each carbon source the antibiotic was formed during growth. In the glucose-lactose medium, the production pattern was biphasic; a marked decrease in the rate of synthesis was associated with depletion of glucose from the medium and a corresponding diauxie pause in growth. Cells of S. venezuelae contained an inducible beta-galactosidase. Induction by lactose was suppressed by glucose. Measurement of the concentration of intracellular adenosine 3',5'-cyclic monophosphate during growth of cultures with glucose or a glucose-lactose mixture as the source of carbon showed no appreciable changes coinciding with depletion of glucose or the onset of chloramphenicol biosynthesis. It is concluded that the cyclic nucleotide does not mediate selective nutrient utilization or control antibiotic biosynthesis in this organism.     

Cloning and location of a gene governing lysine epsilon-aminotransferase, an enzyme initiating beta-lactam biosynthesis in Streptomyces spp.

In actinomycetes that produce beta-lactam antibiotics of the cephem type, lysine epsilon-aminotransferase is the initial enzyme in the conversion of lysine to alpha-aminoadipic acid. We used a two-stage process ("chromosome walking") to screen a lambda library of Streptomyces clavuligerus genomic DNA for fragments that expressed lysine epsilon-aminotransferase activity in S. lividans. Restriction analysis of the cloned DNA confirmed the location of the putative lat gene within the cluster of beta-lactam biosynthesis genes, roughly midway between pcbC, the structural gene for isopenicillin N synthetase, and the putative cefE gene encoding deacetoxycephalosporin C synthetase.     

Control of cell volume in the J774 macrophage by microtubule disassembly and cyclic AMP

We have explored the possibilities that cell volume is regulated by the status of microtubule assembly and cyclic AMP metabolism and may be coordinated with shape change. Treatment of J774.2 mouse macrophages with colchicine caused rapid microtubule disassembly and was associated with a striking increase (from 15-20 to more than 90 percent) in the proportion of cells with a large protuberance at one pole. This provided a simple experimental system in which shape changes occurred in virtually an entire cell population in suspension. Parallel changes in cell volume could then be quantified by isotope dilution techniques. We found that the shape change caused by colchicine was accompanied by a decrease in cell volume of approximately 20 percent. Nocodozole, but not lumicolchicine, caused identical changes in both cell shape and cell volume. The volume loss was not due to cell lysis nor to inhibition of pinocytosis. The mechanism of volume loss was also examined. Colchicine induced a small but reproducible increase in activity of the ouabain-sensitive Na(+), K(+)-dependent ATPase. However, inhibition of this enzyme/transport system by ouabain did not change cell volume nor did it block the colchicines-induced decrease in volume. One the other hand, SITS (4’acetamido, 4-isothiocyano 2,2’ disulfonic acid stilbene), an inhibitor of anion transport, inhibited the effects of colchicines, thus suggesting a role for an anion transport system in cell volume regulation. Because colchicine is known to activate adenylate cyclase in several systems and because cell shape changes are often induced by hormones that elevate cyclic AMP, we also examined the effects of cyclic AMP on cell volume. Agents that act to increase syclic AMP (cholera toxin, which activates adenylate cyclase; IBMX, and inhibitor of phosphodiesterase; and dibutyryl cyclic AMP) all caused a volume decrease comparable to that of colchicine. To define the effective metabolic pathway, we studied two mutants of J774.2, one deficient in adenylate cyclase and the other exhibiting markedly reduced activity of cyclic AMP-dependent protein kinase. Cholera toxin did not produce a volume change in either mutant. Cyclic AMP produced a decrease in the cyclase-deficient line comparable to that in wild type, but did not cause a volume change in the kinase- deficient line. This analysis established separate roles for cyclic AMP and colchicine. The volume decrease induced by cyclic AMP requires the action of a cyclic AMP-dependent protein kinase. Colchicine, on the other hand, induced a comparable volume change in both mutants and wild type, and thus does not require the kinase.     

Biosynthesis of chloramphenicol in Streptomyces sp. 3022a. Identification of p-amino-L-phenylalanine as a product from the action of arylamine synthetase on chorismic acid.

Products obtained from the action of arylamine synthetase on [G-14C]chorismic acid were fractionated by gel filtration and ion exchange column chromatography to yield a partially purified radioactive component with an arylamine function. From its ultraviolet absorption spectrum and thin-layer chromatographic behaviour the product was considered to be p-aminophenylalanine and the identification was confirmed by co-crystallization with an authentic specimen. Specific deamination of the product with L-amino-acid oxidase indicated that it was the L-epimer. These results strengthen previous evidence that arylamine synthetase is at a branch point in the shikimic acid pathway, specifically diverting intermediates to the synthesis of chloramphenicol.