THE METHYLGLYCOSIDES OF THE NATURALLY OCCURRING HEXURONIC ACIDS

Molecular cloning of the structural gene for exopolygalacturonate lyase from Erwinia chrysanthemi EC16 and characterization of the enzyme product

The ability of Erwinia chrysanthemi to cause soft-rot diseases involving tissue maceration in many plants has been linked to the production of endo-pectate lyase E. chrysanthemi EC16 mutant UM1005, however, contains deletions in the pel genes that encode the known endopectate lyases, yet still macerates plant tissues. In an attempt to identify the remaining macerating factor(s), a gene library of UM1005 was constructed in Escherichia coli and screened for pectolytic activity. A clone (pPNL5) was identified in this library that contained the structural gene for an exopolygalacturonate lyase (ExoPL). The gene for ExoPL was localized on a 3.3-kb EcoRV fragment which contained an open reading frame for a 79,500-Da polypeptide. ExoPL was purified to apparent homogeneity from Escherichia coli DH5 alpha (pPNL5) and found to have an apparent molecular weight of 76,000 with an isoelectric point of 8.6. Purified ExoPL had optimal activity between pH 7.5 and 8.0 and could utilize pectate, citrus pectin, and highly methyl-esterified Link pectin as substrates. A PL- ExoPL- mutant of EC16 was constructed that exhibited reduced growth on pectate, but retained pathogenicity on chrysanthemum equivalent to that of UM1005. The results indicate that ExoPL does not contribute to the residual macerating activity of UM1005.

Polysaccharide lyases

Polysaccharide lyases (or eliminases) are a class of enzymes (EC 4.2.2.-) that act to cleave certain activated glycosidic linkages present in acidic polysaccharides. These enzymes act through an eliminase mechanism, rather than through hydrolysis, resulting in unsaturated oligosaccharide products. Acidic polysaccharides are ubiquitous and so are the lyases that degrade them. This review article examines lyases that act on acidic polysaccharides of plant, animal, and microbial origin. These lyases are predominantly of microbial origin and come from a wide variety of both pathogenic and nonpathogenic bacteria and fungi. The lyases discussed include alginate lyase (EC 4.2.2.3), pectin lyase (EC 4.2.2.10), pectate lyase (EC 4.2.2.2), oligogalacturonide lyase (EC 4.2.2.6), exopolygalacturonate lyase (EC 4.2.2.9), chondroitin lyases (EC 4.2.2.4 and EC 4.2.2.5), hyaluronate lyase (EC 4.2.2.1), heparin lyase (EC 4.2.2.7), heparan lyase (EC 4.2.2.8), and other unclassified lyases. This review examines the sources, regulation, purification, and properties of these polysaccharide lyases.

recA is required in the induction of pectin lyase and carotovoricin in Erwinia carotovora subsp. carotovora

Pectin lyase (PNL) and the bacteriocin carotovoricin (CTV) were induced in Erwinia carotovora subsp. carotovora 71 by the DNA-damaging agents mitomycin C, nalidixic acid, and UV light. To determine whether the recA product was involved in the expression of these damage-inducible phenotypes, we cloned the E. carotovora subsp. carotovora recA+ gene, inactivated it by Tn5 insertion, and constructed an E. carotovora subsp. carotovora recA::Tn5 strain by gene replacement via homologous recombination. The RecA- strain was more sensitive to methyl methanesulfonate, nitroquinoline oxide, and UV light than its RecA+ parent. The recA mutation did not affect the production of pectate lyase, polygalacturonase, cellulase, and protease or the ability to cause soft rot of potato tubers. With this mutant, unlike with the RecA+ parent strain, PNL and CTV were not induced by mitomycin C or detected in potato tuber tissue. The RecA+ phenotype, including the inducibility of PNL and CTV, could, however, be restored in the mutant in trans by the recA+ gene from either E. carotovora subsp. carotovora or Escherichia coli. We conclude that, in E. carotovora subsp. carotovora, the recA product is required in the induction of PNL and CTV.

Coordinated action of pectinesterase and polygalacturonate lyase complex of Clostridium multifermentans

The polygalacturonate lyase and pectinesterase activities of Clostridium multifermentans, both produced extracellularly when the organism grows on pectin or polygalacturonate, have been suggested to be associated in a single complex. Both enzymic sites act on their respective substrates by single-chain action patterns, as shown by equivalent release of terminal tritium label and total product throughout the reaction. From these results, the Km and V of the lyase, and the amount of lyase activity present, we calculate the steady-state concentration of lyase substrate expected during action of the two sites on pectin if the sites are independent. No such steady-state concentration of lyase substrate was observed. Therefore, we conclude that the two types of active site act in a coordinated manner; the polysaccharide chain passes from the esterase site to the lyase site without intermediate dissociation and rebinding. This 'molecular disassembly line' constituted by the two sites may represent a system of general significance in synthesis and degradation of biological polymers.

Similarities in the action patterns of exopolygalacturonate lyase and pectinesterase from Clostridium multifermentans

Exopolygalacturonate lyase and pectinesterase from Clostridium multifermentans were assayed simultaneously in the same reaction mixture which contained a highly esterified pectin, polymethyl polygalacturonic acid methyl glycoside. Lyase is specific for unesterified galacturonide residues and cannot degrade this substrate in the absence of the esterase. The rate for esterase was twice the rate for lyase throughout the entire course of the combined reaction. Thus, the molar ratio of the two enzyme activities was the same since the product of the lyase is an unsaturated digalacturonic acid containing two free carboxyl groups. Since clostridial exopolygalacturonate lyase is known to degrade polygalacturonate in a linear manner beginning from the reducing ends of polygalacturonate chains, it was apparent that clostridial pectinesterase must hydrolyze methyl groups in highly esterified pectins with an action pattern similar to that of the lyase. Otherwise it would be impossible for the two enzyme rates to have corresponded on the basis of a 2:1 ratio.