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J L Koch

Publications and source records attributed to J L Koch.

7 recordsLinked to original sources
PubMed1992

Group II introns deleted for multiple substructures retain self-splicing activity.

Group II introns can be folded into highly conserved secondary structures with six major substructures or domains. Domains 1 and 5 are known to play key roles in self-splicing, while the roles of domains 2, 3, 4, and 6 are less clear. A trans assay for domain 5 function has been developed which indicates that domain 5 has a binding site on the precursor RNA that is not predicted from any secondary structure element. In this study, the self-splicing group II intron 5 gamma of the coxI gene of yeast mitochondrial DNA was deleted for various intron domains, singly and in combinations. Those mutant introns were characterized for self-splicing reactions in vitro as a means of locating the domain 5 binding site. A single deletion of domain 2, 3, 4, or 6 does not block in vitro reactions at either splice junction, though the deletion of domain 6 reduces the fidelity of 3' splice site selection somewhat. Even the triple deletion lacking domains 2, 4, and 6 retains some self-splicing activity. The deletion of domains 2, 3, 4, and 6 blocks the reaction at the 3' splice junction but not at the 5' junction. From these results, we conclude that the binding site for domain 5 is within domain 1 and that the complex of 5' exon, domain 1, and domain 5 (plus short connecting sequences) constitutes the essential catalytic core of this intron.

Base Sequence↗
PubMed1990

The Tomato Fruit Cell Wall : II. Polyuronide Metabolism in a Nonsoftening Tomato Mutant.

A nonsoftening tomato (Lycopersicon esculentum L.) variety, dg, was examined to assess the physiological basis for its inability to soften during ripening. Total uronic acid levels, 18 milligrams uronic acid/100 milligrams wall, and the extent of pectin esterification, 60 mole%, remained constant throughout fruit development in this mutant. The proportion of uronic acid susceptible to polygalacturonase in vitro also remained constant. Pretreatment of heat-inactivated dg fruit cell walls with tomato pectinmethylesterase enhances polygalacturonase susceptibility at all ripening stages. Pectinesterase activity of cell wall protein extracts from red ripe dg fruit was half that in extracts from analogous tissue of VF145B. Polygalacturonase activities of cell wall extracts, however, were similar in both varieties. Diffusion of uronic acid from tissue discs of both varieties increased beginning at the turning stage to a maximum of 2.0 milligrams uronic acid released/gram fresh weight at the ripe stage. The increased quantity of hydrolytic products released during ripening suggests the presence of in situ polygalacturonase activity. Low speed centrifugation was employed to induce efflux of uronide components from the cell wall tree space. In normal fruit, at the turning stage, 2.1 micrograms uronic acid/gram fresh weight was present in the eluant after 1 hour, and this value increased to a maximum of 8.2 micrograms uronic acid/gram fresh weight at the red ripe stage. However, centrifuge-aided extraction of hydrolytic products failed to provide evidence for in situ polygalacturonase activity in dg fruit. We conclude that pectinesterase and polygalacturonase enzymes are not active in situ during the ripening of dg fruit. This could account for the maintenance of firmness in ripe fruit tissue.

Journal Article↗
PubMed1989

Tomato fruit cell wall : I. Use of purified tomato polygalacturonase and pectinmethylesterase to identify developmental changes in pectins.

Cell wall isolation procedures were evaluated to determine their effect on the total pectin content and the degree of methylesterification of tomato (Lycopersicon esculentum L.) fruit cell walls. Water homogenates liberate substantial amounts of buffer soluble uronic acid, 5.2 milligrams uronic acid/100 milligrams wall. Solubilization appears to be a consequence of autohydrolysis mediated by polygalacturonase II, isoenzymes A and B, since the uronic acid release from the wall residue can be suppressed by homogenization in the presence of 50% ethanol followed by heating. The extent of methylesterification in heat-inactivated cell walls, 94 mole%, was significantly greater than with water homogenates, 56 mole%. The results suggest that autohydrolysis, mediated by cell wall-associated enzymes, accounts for the solubilization of tomato fruit pectin in vitro. Endogenous enzymes also account for a decrease in the methylesterification during the cell wall preparation. The heat-inactivated cell wall preparation was superior to the other methods studied since it reduces beta-elimination during heating and inactivates constitutive enzymes that may modify pectin structure. This heat-inactivated cell wall preparation was used in subsequent enzymatic analysis of the pectin structure. Purified tomato fruit polygalacturonase and partially purified pectinmethylesterase were used to assess changes in constitutive substrates during tomato fruit ripening. Polygalacturonase treatment of heat-inactivated cell walls from mature green and breaker stages released 14% of the uronic acid. The extent of the release of polyuronides by polygalacturonase was fruit development stage dependent. At the turning stage, 21% of the pectin fraction was released, a value which increased to a maximum of 28% of the uronides at the red ripe stage. Pretreatment of the walls with purified tomato pectinesterase rendered walls from all ripening stages equally susceptible to polygalacturonase. Quantitatively, the release of uronides by polygalacturonase from all pectinesterase treated cell walls was equivalent to polygalacturonase treatment of walls at the ripe stage. Uronide polymers released by polygalacturonase contain galacturonic acid, rhamnose, galactose, arabinose, xylose, and glucose. As a function of development, an increase in the release of galacturonic acid and rhamnose was observed (40 and 6% of these polymers at the mature green stage to 54 and 15% at the red ripe stage, respectively). The amount of galactose and arabinose released by exogenous polygalacturonase decreased during development (41 and 11% from walls of mature green fruit to 11 and 6% at the red ripe stage, respectively). Minor amounts of glucose and xylose released from the wall by exogenous polygalacturonase (4-7%) remained constant throughout fruit development.

Journal Article↗
PubMed1982

Evidence of singlet oxygen participation in the chlorophyll-sensitized photooxidation of indoleacetic Acid.

Chlorophyll-sensitized photooxidation of indoleacetic acid (IAA)-with chlorophyll extracted from Pisum sativum L. cv. Alaska W.R.-was determined in the presence of deuterium oxide and known quenchers of singlet oxygen ((1)O(2)) to explore the involvement of (1)O(2) in the reaction. O(2) uptake was measured in light in a buffered aqueous micellar system containing Triton X-100, KCl, chlorophyll, and IAA. The rate of O(2) uptake was zero in darkness. The reaction was stimulated by deuterium oxide and inhibited by sodium azide indicating that (1)O(2) participated in IAA photooxidation. Both mannitol and superoxide dismutase failed to inhibit O(2) uptake suggesting that neither the hydroxyl radical nor the superoxide anion played a significant role in the reaction.

Journal Article↗