Molecular cloning of complementary DNA encoding the lignin-forming peroxidase from tobacco: Molecular analysis and tissue-specific expression

Plant peroxidases play a major role in lignin formation and wound healing and are believed to be involved in auxin catabolism and defense to pathogen attack. The function of the anionic peroxidase isozymes is best understood in tobacco. These isozymes catalyze the formation of the lignin polymer and form rigid cross-links between lignin, cellulose, and extensin in the secondary plant cell wall. We report the purification of the anionic peroxidase isozymes from tobacco and their partial amino acid sequence. An oligonucleotide probe deduced from the amino acid sequence was used to screen a tobacco leaf cDNA library and a 1200-base-pair cDNA clone was isolated and sequenced in its entirety. The predicted amino acid sequence revealed a 22-amino acid signal peptide and a 302-amino acid mature protein (M(r), 32,311). The amino acid sequence was compared to that of the cationic peroxidases from horseradish and turnip and was found to be 52% and 46% homologous, respectively. By RNA blot analysis, the messenger for the tobacco isozyme was found to be abundant in stem tissue while expressed at very low levels in leaf and root tissue. Four distinguishable copies of the gene were found on genomic DNA blots. The gene copy number may reflect the allotetraploid nature of Nicotiana tabacum.

Response to neonatal anesthesia: effect of sex on anatomical and behavioral outcome

Numerous studies have documented the consequences of exposure to anesthesia in models of term and post-term infants, evaluating the incidence of cell loss, physiological alterations and cognitive dysfunction. However, surprisingly few studies have investigated the effect of anesthetic exposure on outcomes in newborn rodents, the developmental equivalent of premature human infants. This is critical given that one out of every eight babies born in the United States is premature, with an increased prevalence of surgical procedures required in these individuals. Also, no studies have investigated if the genetic sex of the individual influences the response to neonatal anesthesia. Using the newborn rat as the developmental equivalent of the premature human, we documented the effect of a single bout of exposure to either the inhalant isoflurane or the injectable barbiturate phenobarbital on hippocampal anatomy, hippocampal dependent behavioral performance and normal developmental endpoints in male and female rats. While both forms of anesthesia led to significant decrements in cognitive abilities, along with a significant reduction in volume and neuron number in the hippocampus in adulthood, the decrements were significantly greater in males than in females. Interestingly, the deleterious effects of anesthesia were manifest on developmental measures including surface righting and forelimb grasp, but were not evident on basic physiological parameters including body weight or suckling. These findings point to the hazardous effects of exposure to anesthesia on the developing CNS and the particular sensitivity of males to deficits.

Selenium and immunocompetence in patients with head and neck cancer

This randomized double-blind placebo-controlled study aimed to determine whether oral intake of 200 microg/d of sodium selenite, a dose within the safe and adequate daily intake (50-200 microg/d) recommended by the U.S. Food and Nutrition Board, will abrogate depressed or enhance normal-level immune functions of patients receiving therapy for squamous cell carcinoma of the head and neck. Subjects were given one selenium/placebo tablet/d for 8 wk, beginning on the day of their first treatment for the disease (e.g., surgery, radiation, or surgery and radiation) and their immune functions were monitored. Supplementation with selenium (Se) during therapy resulted in a significantly enhanced cell-mediated immunue responsiveness, as reflected in the ability of the patient's lymphocytes to respond to stimulation with mitogen, to generate cytotoxic lymphocytes, and to destroy tumor cells. The enhanced responsiveness was evident during therapy and following conclusion of therapy. In contrast, patients in the placebo arm of the study showed a decline in immune responsiveness during therapy, which was followed, in some patients, by an enhancement, but the responses of the group remained significantly lower than baseline values. The data also show that at baseline, patients entered in the study had significantly lower plasma Se levels than healthy individuals, and patients in stage I or II of disease had significantly higher plasma selenium levels than patients in stage III or IV of disease.

A common mechanism controls the life cycle and architecture of plants

The overall aerial architecture of flowering plants depends on a group of meristematic cells in the shoot apex. We demonstrate that the Arabidopsis TERMINAL FLOWER 1 gene has a unified effect on the rate of progression of the shoot apex through different developmental phases. In transgenic Arabidopsis plants which ectopically express TERMINAL FLOWER 1, both the vegetative and reproductive phases are greatly extended. As a consequence, these plants exhibit dramatic changes in their overall morphology, producing an enlarged vegetative rosette of leaves, followed by a highly branched inflorescence which eventually forms normal flowers. Activity of the floral meristem identity genes LEAFY and APETALA 1 is not directly inhibited by TERMINAL FLOWER 1, but their upregulation is markedly delayed compared to wild-type controls. These phenotypic and molecular effects complement those observed in the tfl1 mutant, where all phases are shortened. The results suggest that TERMINAL FLOWER 1 participates in a common mechanism underlying major shoot apical phase transitions, rather than there being unrelated mechanisms which regulate each specific transition during the life cycle.

A common mechanism controls the life cycle and architecture of plants

The overall aerial architecture of flowering plants depends on a group of meristematic cells in the shoot apex. We demonstrate that the Arabidopsis TERMINAL FLOWER 1 gene has a unified effect on the rate of progression of the shoot apex through different developmental phases. In transgenic Arabidopsis plants which ectopically express TERMINAL FLOWER 1, both the vegetative and reproductive phases are greatly extended. As a consequence, these plants exhibit dramatic changes in their overall morphology, producing an enlarged vegetative rosette of leaves, followed by a highly branched inflorescence which eventually forms normal flowers. Activity of the floral meristem identity genes LEAFY and APETALA 1 is not directly inhibited by TERMINAL FLOWER 1, but their upregulation is markedly delayed compared to wild-type controls. These phenotypic and molecular effects complement those observed in the tfl1 mutant, where all phases are shortened. The results suggest that TERMINAL FLOWER 1 participates in a common mechanism underlying major shoot apical phase transitions, rather than there being unrelated mechanisms which regulate each specific transition during the life cycle.

Characterization of Antisense Transformed Plants Deficient in the Tobacco Anionic Peroxidase

On the basis of the biological compounds that they metabolize, plant peroxidases have long been implicated in plant growth, cell wall biogenesis, lignification, and host defenses. Transgenic tobacco (Nicotiana tabacum L.) plants that underexpress anionic peroxidase were generated using antisense RNA. The antisense RNA was found to be specific for the anionic isoenzyme and highly effective, reducing endogenous transcript levels and total peroxidase activity by as much as 1600-fold. Antisense-transformed plants appeared normal at initial observation; however, growth studies showed that plants with reduced peroxidase activity grow taller and flower sooner than control plants. In contrast, previously transformed plants overproducing anionic peroxidase were shorter and flowered later than controls. Axillary buds were more developed in antisense-transformed plants and less developed in plants overproducing this enzyme. It was found that the lignin content in leaf, stem, and root was unchanged in antisense-transformed plants, which does not support a role for anionic peroxidase in the lignification of secondary xylem vessels. However, studies of wounded tissue show some reduction in wound-induced deposition of lignin-like polymers. The data support a possible role for tobacco anionic peroxidase in host defenses but not without a reduction in growth potential.

Regulation of the Accumulation and Reduction of Nitrate by Nitrogen and Carbon Metabolites in Maize Seedlings

The accumulation and reduction of nitrate in the presence of the nitrogen metabolites asparagine (Asn) and glutamine (Gln) and the carbon metabolite sucrose (Suc) were examined in maize (Zea mays L.) seedlings in an attempt to separate their effects on the nitrate uptake system and the nitrate reduction system. After 8 h of exposure to nitrate in the presence of 1 mM Asn, tissue nitrate accumulation was reduced at 250 [mu]M external nitrate, but not at 5 mM Asn. The induction of nitrate reductase (NR) activity was reduced at both external nitrate concentrations. In the presence of 1 mM Gln or 1% Suc, tissue nitrate concentration was not significantly altered, but the induction of root NR activity was reduced or enhanced, respectively. The induction of root nitrite reductase (NiR) activity was also reduced in the presence of Asn or Gln and enhanced in the presence of Suc. Transcript levels of NR and NiR in roots were reduced in the presence of the amides and enhanced in the presence of Suc. When Suc was present in combination with either amide, there was complete relief from the inhibition of NiR transcription observed in the presence of amide alone. In the case of NR, however, this relief from inhibition was negligible. The inhibition of the induction of NR and NiR activities in the presence of Gln and Asn is a direct effect and is not the result of altered nitrate uptake in the presence of these metabolites.

Structure and regulation of ferredoxin-dependent glutamase synthase from Arabidopsis thaliana. Cloning of cDNA expression in different tissues of wild-type and gltS mutant strains, and light induction

Ferredoxin (Fd)-dependent glutamate synthase is present in green leaves, etiolated leaves, shoots and roots of Arabidopsis thaliana (ecotype Columbia). In photosynthetic green leaves and shoots, Fd-dependent glutamate synthase accounts for more than 96% of the total glutamate synthase activity in vitro with the remaining activity derived from an enzyme that uses NADH as the electron donor. In etiolated leaves and roots, Fd-dependent glutamate synthase is 3-4-fold less active than in green leaves, but represents 70-85% of the total glutamate synthase activity in these tissues. Fd-dependent glutamate synthase is detected as a single peptide of 165 kDa on a western blot of green leaf and shoot tissues, and this Fd-dependent glutamate synthase polypeptide is 3-4-fold less abundant in etiolated leaves and roots. In these non-photosynthetic tissues, there is a higher activity of NADH-dependent glutamate synthase. The A. thaliana gltS mutant (strain CS254) contains only 1.7% and 17.5% of the wild-type Fd-dependent glutamate synthase activity in leaves and roots, respectively. Western blots indicate that the Fd-dependent glutamate synthase peptide of 165 kDa is absent from leaves and roots of the gltS mutant. In contrast, NADH-dependent glutamate synthase activity in leaves and roots is unaffected. During illumination of wild-type dark-grown leaves for 72 h, the levels of Fd-dependent glutamate synthase protein and its activity increased threefold to levels equivalent to those in green leaves. In contrast, NADH-dependent glutamate synthase activity decrease twofold during illumination. The complete nucleotide sequence of the complementary DNA for A. thaliana Fd-dependent glutamate synthase has been determined. Analysis of the amino acid sequence deduced from the complete cDNA sequence (5178 bp) has revealed that A. thaliana Fd-dependent glutamate synthase is synthesized as a 1648-amino-acid precursor protein (180090 Da) which consists of a 131-amino-acid transit peptide (14603 Da) and a 1517-amino-acid mature peptide (165487 Da). The A. thaliana Fd-dependent glutamate synthase has a high similarity to maize Fd-dependent glutamate synthase (83%) and to the analogous region of NADH-dependent glutamate synthase (42%) and NADPH-dependent glutamate synthases (40-43%) from different organisms. The A. thaliana Fd-dependent glutamate synthase contains the purF-type glutamine-amido-transfer domain as well as flavin and iron-sulfur-cluster-binding domains. The deduced primary structures of A. thaliana Fd-dependent glutamate synthase and of glutamate synthases from other organisms indicate that Fd-dependent glutamate synthase may have evolved from bacterial NADPH-dependent glutamate synthase. The cDNA hybridized to RNA of about 5.3 kb from different tissues of A. thaliana. A high steady-state level of Fd-dependent glutamate synthase mRNA is found in photosynthetic green leaves and shoots, and roots contain less mRNA for Fd-dependent glutamate synthase. In the gltS mutant, there are twofold and fourfold lower levels of Fd-dependent glutamate synthase mRNA in leaves and roots, respectively, relative to those in wild-type A. thaliana. Under continuous illumination of dark-grown leaves, the Fd-dependent glutamate synthase mRNA is induced twofold to a level equivalent to that in green leaves.

Control of inflorescence architecture in Antirrhinum

Flowering plants exhibit two types of inflorescence architecture: determinate and indeterminate. The centroradialis mutation causes the normally indeterminate inflorescence of Antirrhinum to terminate in a flower. We show that centroradialis is expressed in the inflorescence apex a few days after floral induction, and interacts with the floral-meristem-identity gene floricaula to regulate flower position and morphology. The protein CEN is similar to animal proteins that associate with lipids and GTP-binding proteins. We propose a model for how different inflorescence structures may arise through the action and evolution of centroradialis.

Extracranial repair of cerebrospinal fluid fistulas: technique and results in 37 patients

Although neurosurgeons have traditionally preferred intracranial repair for the management of cerebrospinal fluid (CSF) fistulas, this approach is associated with the complications of a craniotomy, anosmia, and a high incidence of recurrent fistulas. Extracranial repair, on the other hand, produces no central nervous system morbidity, preserves olfaction, and is associated with a low incidence of recurrence. Although there have been several reports of extracranial repair of CSF fistulas by otorhinolaryngologists, this approach has received scant mention in the neurosurgical literature. We report here our experience with 37 patients with CSF rhinorrhea or otorrhea who underwent extracranial repair. The etiology of the fistula was postoperative in 22, traumatic in 6, and spontaneous in 9. The fistulas were repaired using one of four techniques: external ethmoid-sphenoid in 18 patients, transmastoid in 9, transseptosphenoid in 7, and osteoplastic frontal sinusotomy in 3. In 32 of the 37 patients (86%) the fistulas were successfully repaired with the initial procedure. Of the 5 patients requiring a second operation, the fistula was successfully closed in 4 for an overall success rate of 97%. Complications were few and consisted of a transient facial paresis in a patient undergoing transmastoid repair and one death from meningitis. The authors conclude that because of low morbidity and mortality and a high success rate in closing fistulas, extracranial repair is the preferred technique for the operative management of CSF rhinorrhea and otorrhea.

Peroxidase-Induced Wilting in Transgenic Tobacco Plants

Peroxidases are a family of isoenzymes found in all higher plants. However, little is known concerning their role in growth, development, or response to stress. Plant peroxidases are heme-containing monomeric glycoproteins that utilize either H2O2 or O2 to oxidize a wide variety of molecules. To obtain more information on possible in planta functions of peroxidases, we have used a cDNA clone for the primary isoenzyme form of peroxidase to synthesize high levels of this enzyme in transgenic plants. We were able to obtain Nicotiana tabacum and N. sylvestris transformed plants with peroxidase activity that is 10-fold higher than in wild-type plants by introducing a chimeric gene composed of the cauliflower mosaic virus 35S promoter and the tobacco anionic peroxidase cDNA. The elevated peroxidase activity was a result of increased levels of two anionic peroxidases in N. tabacum, which apparently differ in post-translational modification. Transformed plants of both species have the unique phenotype of chronic severe wilting through loss of turgor in leaves, which was initiated at the time of flowering. The peroxidase-induced wilting was shown not to be an effect of diminished water uptake through the roots, decreased conductance of water through the xylem, or increased water loss through the leaf surface or stomata. Possible explanations for the loss of turgor, and the significance of these types of experiments in studying isoenzyme families, are discussed.

Peroxidase-Induced Wilting in Transgenic Tobacco Plants

Peroxidases are a family of isoenzymes found in all higher plants. However, little is known concerning their role in growth, development, or response to stress. Plant peroxidases are heme-containing monomeric glycoproteins that utilize either H2O2 or O2 to oxidize a wide variety of molecules. To obtain more information on possible in planta functions of peroxidases, we have used a cDNA clone for the primary isoenzyme form of peroxidase to synthesize high levels of this enzyme in transgenic plants. We were able to obtain Nicotiana tabacum and N. sylvestris transformed plants with peroxidase activity that is 10-fold higher than in wild-type plants by introducing a chimeric gene composed of the cauliflower mosaic virus 35S promoter and the tobacco anionic peroxidase cDNA. The elevated peroxidase activity was a result of increased levels of two anionic peroxidases in N. tabacum, which apparently differ in post-translational modification. Transformed plants of both species have the unique phenotype of chronic severe wilting through loss of turgor in leaves, which was initiated at the time of flowering. The peroxidase-induced wilting was shown not to be an effect of diminished water uptake through the roots, decreased conductance of water through the xylem, or increased water loss through the leaf surface or stomata. Possible explanations for the loss of turgor, and the significance of these types of experiments in studying isoenzyme families, are discussed.

Nitrate effects on nitrate reductase activity and nitrite reductase mRNA levels in maize suspension cultures

Nitrate reductase (NR) activity and nitrite reductase (NiR) mRNA levels were monitored in Black Mexican Sweet maize (Zea mays L.) suspension cultures after the addition of nitrate. Maximal induction occurred with 20 millimolar nitrate and within 2 hours. Both NR and NiR mRNA were transiently induced with levels decreasing after the 2 hours despite the continued presence of nitrate in the medium. Neither ammonia nor chlorate prevented the induction of NR. Furthermore, removal of nitrate, followed by its readdition 22 to 48 hours later, did not result in reinduction of activity or message. NR was synthesized de novo, since cycloheximide completely blocked its induction. Cycloheximide had no effect on the induction of NiR mRNA or on the transient nature of its induction. These results are similar to those reported previously for maize seedlings.

Transient Accumulation of Nitrite Reductase mRNA in Maize following the Addition of Nitrate

Expression of the gene coding for nitrite reductase (NiR) is induced upon the addition of nitrate. We have analyzed this induction process in hydroponically grown maize (Zea mays L.) seedlings where the level of nitrate in the medium can be easily manipulated. There is a rapid induction of NiR mRNA upon addition of nitrate, increasing first in the roots and then in the leaves. The rapidity of the response depends on the nitrate concentration and the growth medium. However, the general pattern of expression is the same: the mRNA level increases, reaches a maximum, and then decreases, despite the fact that the nitrate concentration in the medium remains constant. This decline in mRNA level can be quite rapid, particularly in root tissue. If the nitrate is given as a pulse, the mRNA levels decrease even more rapidly. It is clear that the NiR mRNA is short-lived, with a half-life in the roots of less than 30 minutes. The NiR protein level, on the other hand, increases gradually somewhat after the increase in mRNA and remains at high levels at least for 24 hours after the addition of nitrate.

Molecular cloning of complementary DNA encoding maize nitrite reductase: molecular analysis and nitrate induction

Complementary DNA has been isolated that codes for maize nitrite reductase (NiR) by using the corresponding spinach gene (E Back et al. 1988 Mol Gen Genet 212:20-26) as a heterologous probe. The sequences of the complementary DNAs from the two species are 66% homologous while the deduced amino acid sequences are 86% similar when analogous amino acids are included. A high percentage of the differences in the DNA sequences is due to the extremely strong bias in the corn gene to have a G/C base in the third codon position with 559/569 codons ending in a G or C. Using a hydroponic system, maize seedlings grown in the absence of an exogenous nitrogen source were induced with nitrate or nitrite. Nitrate stimulated a rapid induction of the NiR mRNA in both roots and leaves. There is also a considerable induction of this gene in roots upon the addition of nitrite, although under the conditions used the final mRNA level was not as high as when nitrate was the inducer. There is a small but detectable level of NiR mRNA in leaves prior to induction, but no constitutive NiR mRNA can be seen in the roots. Analysis of genomic DNA supports the notion that there are at least two NiR genes in maize.

Isolation of cDNA clones coding for spinach nitrite reductase: complete sequence and nitrate induction

The main nitrogen source for most higher plants is soil nitrate. Prior to its incorporation into amino acids, plants reduce nitrate to ammonia in two enzymatic steps. Nitrate is reduced by nitrate reductase to nitrite, which is further reduced to ammonia by nitrite reductase. In this paper, the complete primary sequence of the precursor protein for spinach nitrite reductase has been deduced from cloned cDNAs. The cDNA clones were isolated from a nitrate-induced cDNA library in two ways: through the use of oligonucleotide probes based on partial amino acid sequences of nitrite reductase and through the use of antibodies raised against purified nitrite reductase. The precursor protein for nitrite reductase is 594 amino acids long and has a 32 amino acid extension at the N-terminal end of the mature protein. These 32 amino acids most likely serve as a transit peptide involved in directing this nuclear-encoded protein into the chloroplast. The cDNA hybridizes to a 2.3 kb RNA whose steady-state level is markedly increased upon induction with nitrate.

Tissue specificity of tobacco peroxidase isozymes and their induction by wounding and tobacco mosaic virus infection

Peroxidases (EC 1.11.1.7) have been implicated in the responses of plants to physical stress and to pathogens, as well as in a variety of cellular processes including cell wall biosynthesis. Tissue samples from leaf, root, pith, and callus of Nicotiana tabacum were assayed for specific peroxidase isozymes by analytical isoelectric focusing. Each tissue type was found to exhibit a unique isozyme fingerprint. Root tissue expressed all of the detectable peroxidase isozymes in the tobacco plant, whereas each of the other tissues examined expressed a different subset of these isozymes. In an effort to determine which peroxidase isozymes from Nicotiana tabacum are involved in cell wall biosynthesis or other normal cellular functions and which respond to stress, plants were subjected to either wounding or infection with tobacco mosaic virus. Wounding the plant triggered the expression of several cationic isozymes in the leaf and both cationic and anionic isozymes in pith tissue. Maximum enzyme activity was detected at 72 hours after wounding, and cycloheximide treatment prevented this induction. Infection of tobacco with tobacco mosaic virus induced two moderately anionic isozymes in the leaves in which virus was applied and also systemically induced in leaves which were not inoculated with virus.

Synthesis of a wheat storage protein subunit in Escherichia coli using novel expression vectors

Useful plasmid expression vectors have been constructed which allow the synthesis of beta-galactosidase (betaG) fusion polypeptides or of polypeptides specified by cDNA clones in Escherichia coli hosts. A foreign DNA fragment can be inserted in any one of the three reading frames at the unique EcoRI, BamHI or SmaI sites immediately after the initiation codon. The cloned foreign gene is under the control of the lac promoter. Using a cDNA clone that encodes part of a wheat storage protein [a high-Mr (HMW) glutenin subunit] synthesis of a glutenin-beta G fusion protein was demonstrated. Synthesis of the glutenin polypeptide, not fused to beta G, was achieved by replacing the lacZYA genes with a stop codon.

Oxygen and carbon dioxide transport in the blood of the muskrat (Ondatra zibethica)

We have investigated the oxygen and carbon dioxide transport properties of a small diving mammal, the muskrat (Ondatra zibethica), where the hemoglobin primary structure has been established by Duffy et al. (1978). While whole blood oxygen capacity, the Haldane effect and the buffer capacity are not different compared to non-diving mammals of similar size, the Bohr effect and the oxygen affinity are increased. The oxygen half saturation pressure (P50) was 26.1 mm Hg (3.5 kPa) at pH 7.4, and the Bohr effect -0.66 (related to plasma pH) and -1.07 related to cell pH. The high affinity of muskrat blood is caused by a comparatively small effect of 2,3 DPG and CO2 on muskrat hemoglobin, that is accentuated through a relatively low concentration of 2,3-DPG in the muskrat red cell. The increased Bohr effect is caused primarily through the pronounced pH dependence of oxygen-linked binding of 2,3-DPG. The weak interaction of muskrat hemoglobin with 2,3-DPG is not caused by substitutions at the binding site.

RNA polymerase binding sites in lambdaplac5 DNA

The in vitro binding of the Escherichia coli RNA polymerase (nucleosidetriphosphate:RNA nucleotidyltransferase; EC 2.7.7.6) to fragments of lambdaplac5 DNA generated by restriction endonucleases HindII and HindIII has been studied by a filter binding technique. The results are consistent with RNA polymerase binding at p(R)', the INT promoter (p(I)), several sites in the b2 region, the mis promoter, the oop promoter (or p(O)), and p(rm). Binding was also observed on some fragments that are not known to contain active promoters, including the fragment from the cIII-t(L) region. Some of these binding reactions might also be explained by interaction of RNA polymerase with termination sites. Additional polymerase binding sites have been detected by examining which HindII and HindIII sites were not cleaved when digestion was performed after RNA polymerase had been bound to the DNA. This technique revealed polymerase binding at p(L), at p(R), at a site between R and cos, and at a site at the junction of the gamma and cIII-t(L) fragments. A comparison of the location of polymerase binding fragments with the partial denaturation map of the lambda genome indicates that RNA polymerase binding sites are located within A-T rich regions. It is suggested that RNA polymerase binding is a function both of specific sequences (where recognition occurs) and of the base composition of the surrounding regions (which affects the stability of the helix at the specific site).

The relationship between anion exchange and net anion flow across the human red blood cell membrane

The conductive (net) anion permeability of human red blood cells was determined from net KCl or K2SO4 effluxes into low K+ media at high valinomycin concentrations, conditions under which the salt efflux is limited primarily by the net anion permeability. Disulfonic stilbenes, inhibitors of anion exchange, also inhibited KCl or K2SO4 efflux under these conditions, but were less effective at lower valinomycin concentrations where K+ permeability is the primary limiting factor. Various concentrations of 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) had similar inhibitory effects on net and exchange sulfate fluxes, both of which were almost completely DIDS sensitive. In the case of Cl-, a high correlation was also found between inhibition of net and exchange fluxes, but in this case about 35% of the net flux was insensitive to DIDS. The net and exchange transport processes differed strikingly in their anion selectivity. Net chloride permeability was only four times as high as net sulfate permeability, whereas chloride exchange is over 10,000 times faster than sulfate exchange. Net OH-permeability, determined by an analogous method, was over four orders of magnitude larger than that of Cl-, but was also sensitive to DIDS. These data and others are discussed in terms of the possibility that a common element may be involved in both net and exchange anion transport.