Inhibition of electron transport in maize mitochondria by Helminthosporium maydis race T pathotoxin

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Expression in Saccharomyces cerevisiae of a gene associated with cytoplasmic male sterility from maize: respiratory dysfunction and uncoupling of yeast mitochondria

We asked whether the mitochondrial T-urf13 gene, associated with the male sterility phenotype of T cytoplasm in maize, can be expressed in Saccharomyces cerevisiae and whether this expression can mimic the effects observed in maize. We introduced the universal code equivalent of the T-urf13 gene into the S. cerevisiae nucleus by transformation and directed its translation product into mitochondria by means of a fusion with the targeting presequence from Neurospora crassa AT-Pase subunit 9. We show that expression of the universal code equivalent of the T-urf13 gene in the yeast nucleus does indeed mimic its effects in maize: respiratory growth of yeast is inhibited, respiration-deficient cytoplasmic mutants accumulate and NADH oxidation of isolated mitochondria is uncoupled. All these effects are observed only if the mitochondrial targeting peptide and methomyl or HmT toxin are present.

A rapid, single leaf assay for detecting the presence of 'S'-male-sterile cytoplasm in maize

A simple, rapid, and reproducible assay is described for determining unambiguously the presence of S-type cytoplasm in male-sterile and male-fertile (restored) maize lines. Because the assay requires only 0.5 g leaf segment per sample, it is a single plant assay and the plant is not destroyed. Plants at any developmental stage can be used. The assay involves a 30 sec homogenization, 20 min centrifugation, one hour lysis, overnight agarose electrophoresis, 30 min gel staining, and photography of the gel to produce a result in much less than 24 hr. Many samples can be assayed simultaneously. The various assay methods available for classifying maize cytoplasms are compared and discussed.

Alterations in mitochondria associated with cytoplasmic and nuclear genes concerned with male sterility in maize

Mitochondria isolated from etiolated shoots of a range of maize genotypes with the "Texas" cytoplasm conferring cytoplasmically-inherited male sterility, are sensitive to a pathotoxin isolated from Helmintho-sporium maydis, race T. The pathotoxin inhibits oxidation of α ketoglutarate and malate and stimulates NADH oxidation. The time taken for the pathotoxin to induce these changes is a measure of the sensitivity of the mitochondria to the pathotoxin. A range of nine different pairs of genotypes, each pair differing principally in the presence of nuclear male fertility restorer alleles has been compared in their sensitivity to pathotoxin. In every case the line carrying the restorer alleles is more resistant to the pathotoxin. The restored genotypes can be quantitatively arranged into groups which correspond to the four different sources of the restorer genes in these lines. It is suggested that the restorer genes cause changes in mitochondria, which modify the functional aberration introduced by the cytoplasmically-inherited mutation causing sterility.