Nucleic acid independent synthesis of peptides

Two enzymes involved in biosynthesis of the host-selective phytotoxin HC-toxin

Cochliobolus carbonum race 1 produces a cyclic tetrapeptide HC-toxin, which is necessary for its exceptional virulence on certain varieties of maize. Previous genetic analysis of HC-toxin production by the fungus has indicated that a single genetic locus controls HC-toxin production. Enzymes involved in the biosynthesis of HC-toxin have been sought by following the precedents established for the biosynthetic enzymes of cyclic peptide antibiotics. Two enzymatic activities from C. carbonum race 1 were found, a D-alanine- and an L-proline-dependent ATP/PP(i) exchange, which by biochemical and genetic criteria were shown to be involved in the biosynthesis of HC-toxin. These two activities were present in all tested race 1 isolates of C. carbonum, which produce HC-toxin, and in none of the tested race 2 and race 3 isolates, which do not produce the toxin. In a genetic cross between two isolates of C. carbonum differing at the tox locus, all tox(+) progeny had both activities, and all tox(-) progeny lacked both activities.

Enzymes acting on peptides containing D-amino acid

Mainly microorganisms but only a few higher organisms are presently known to express enzymes that hydrolyze peptides containing D-amino acids. These enzymes can be involved in proceedings at the bacterial cell wall, in either assembly or modification, and thus cause resistance to glycopeptide antibiotics, or mediate resistance against beta-lactam antibiotics. In other cases the in vivo function is still unknown. New enzymes screened from nature, such as D-aminopeptidase, D-amino acid amidase, alkaline D-peptidase or D-aminoacylase, offer potential application in the production of D-amino acids, the synthesis of D-amino acid oligomers by promoting the reversed reaction under appropriate conditions, or in the field of semi-synthetic antibiotics.

Structure elucidation and 3D solution conformation of the antibiotic enduracidin determined by NMR spectroscopy and molecular dynamics

Enduracidin and ramoplanin belong to the large family of cyclodepsipeptide antibiotics, highly effective against Gram-positive bacteria. The primary and 3D solution structure of ramoplanin is already well known, and the primary structure of enduracidin has been determined by a combination of chemical and NMR spectroscopic methods. Both antibiotics share a similar peptide core of 17 amino acids and differ mainly in the length of the acyl chain and the presence of two D-mannose moieties in ramoplanin. Based on the high sequence homology with ramoplanin, the structure in solution of enduracidin is modeled as a cyclic peptide. The tertiary structure thus obtained was refined through molecular dynamics (MD) simulation, in which the interatomic NOE-derived distance restraints were imposed. MD simulations yielded a family of representative 3D structures (RMSD = 0.89), which highlighted a backbone geometry similar to that of ramoplanin in its beta-hairpin arrangement. In contrast, enduracidin displays a different arrangement of the side-chain and of the residues forming the hydrophobic core.

Substrate specificity of L-delta-(alpha-aminoadipoyl)-L-cysteinyl-D-valine synthetase from Cephalosporium acremonium: demonstration of the structure of several unnatural tripeptide products

Potential substrates for L-delta-(alpha-aminoadipoyl)-L-(cysteinyl)-D-valine (ACV) synthetase were initially identified using both the amino-acid-dependent ATP<-->pyrophosphate exchange reaction catalysed by the enzyme and the incorporation of 14C-radiolabelled cysteine and valine into potential peptide products. S-Carboxymethylcysteine was an effective substitute for alpha-aminoadipate and both allylglycine and vinylglycine could substitute for cysteine, indicating that the thiol group of cysteine is not essential for peptide formation. L-allo-Isoleucine but not L-isoleucine substituted effectively for valine. The structures of the presumed peptide products derived from these amino acids were confirmed by combined use of electrospray-ionization m.s. (e.s.m.s.) and 1H n.m.r. These results clearly indicate that, in common with other peptide synthetases, but in contrast with ribosomal peptide synthesis, ACV synthetase has a relatively broad substrate specificity.

[Biosynthesis of peptides: a non-ribosomal system]

The biosynthesis of peptides in nonribosomal systems is accomplished by complex multienzymes. These multienzymes assemble the required template for the construction of each natural product in the form of linearly coupled modules. This organization principle permits the integration of multistep synthetic processes on a single macromolecule.

Nonribosomal biosynthesis of peptide antibiotics

Peptide antibiotics are known to contain non-protein amino acids, D-amino acids, hydroxy acids, and other unusual constituents. In addition they may be modified by N-methylation and cyclization reactions. Their biosynthetic origin has been connected in many cases to an enzymatic system referred to as the 'thiotemplate multienzymic mechanism'. This mechanism includes the activation of the constituent residues as adenylates on the enzymic template, the acylation of specific template thiol groups, epimerization or N-methylation at this thioester stage, and polymerization in the sequence directed by the multienzymic structure with the aid of 4'-phosphopantetheine as a cofactor, including possible cyclization or terminal modification reactions. The reaction sequences leading to gramicidin S, tyrocidine, cyclosporine, bacitracin, polymyxin, actinomycin, enniatin, beauvericin, delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine and linear gramicidin are discussed. The structures of the multienzymes, their genetic organization, the biological functions of these peptides and results on related systems are discussed.

Streptomyces genes involved in biosynthesis of the peptide antibiotic valinomycin

We have identified genes from Streptomyces levoris A-9 involved in the biosynthesis of the peptide antibiotic valinomycin. Two segments of chromosomal DNA were recovered from genomic libraries, constructed by using the low-copy-number plasmid pIJ922, by complementation of valinomycin-deficient (vlm) mutants of S. levoris A-9. One set of plasmids restored valinomycin production to only one mutant, that carrying vlm-1, whereas a second set of plasmids restored productivity to seven vlm mutants, those carrying vlm-2 through vlm-8. Additional complementation studies using subcloned restriction enzyme fragments showed that the vlm-1+ gene was contained within a 2.5-kilobase (kb) DNA region, whereas alleles vlm-2+ through vlm-8+ were contained in a 12-kb region, representing at least three genes. Physical mapping experiments based on the isolation of cosmid clones showed that the two vlm loci were 50 to 70 kb apart. Southern hybridization experiments demonstrated that the vlm-2+ gene cluster was highly conserved among other valinomycin-producing Streptomyces strains, whereas the vlm-1+ gene was ubiquitous among Streptomyces species tested. Increasing the copy number of the vlm-2+ gene cluster in S. levoris A-9 by the introduction of low-copy-number recombinant plasmids resulted in a concomitant increase in the level of valinomycin production.

Cyclopeptine synthetase activity in surface cultures of Penicillium cyclopium

Cyclopeptine synthetase, the key enzyme of benzodiazepine alkaloid biosynthesis in Penicillium cyclopium forms cyclo-(anthranoyl-phenylalanyl) from anthranilic acid, L-phenylalanine, the methyl group of L-methionine and ATP. The following in vitro measurable partial activities of the enzyme system were followed during the development of P. cyclopium: anthranilic acid and L-phenylalanine adenylyltransferase activities, and the ability for thioester-binding of L-phenylalanine to the enzyme protein. These activities became measurable at the beginning of the idiophase and reached a maximum 6 days after inoculation, i.e., the pattern of activity was similar to that of the other enzymes participating in the biosynthesis of the benzodiazepine alkaloids indicating that the activities of all enzymes of the pathway were coordinatedly expressed. Inhibitor experiments indicated that 48-55 h after inoculation a preprotein of anthranilic acid adenylyltransferase was formed, which later on became activated by a hitherto unknown mechanism.