Isolation of a novel human homologue of the gene coding for echinoderm microtubule-associated protein (EMAP) from the Usher syndrome type 1a locus at 14q32

Usher syndrome type 1 (USH1) is an autosomal recessive, genetically heterogeneous disorder causing severe congenital deafness, retinitis pigmentosa, and vestibular dysfunction. The USHla locus located on 14q32 has been linked to the genetic markers D14S250 and D14S78. Using D14S250 and D14S78, we have isolated two nonchimeric YACs, 878g10 and 844g2, and a single BAC (135i20) and PAC (194e17) clone and have arranged them into a contig spanning over the D14S250 and D14S78 markers. The analysis of the YACs, BAC, and PAC revealed that the physical distance between D14S250 and D14S78 is less than 25 kb. Iterative cDNA library screening initiated with the EST 219670 found in the vicinity of the D14S78 marker yielded a cDNA contig. The nucleotide sequence of the cDNA encodes a protein of 717 amino acids in length, showing a high level of homology to the Echinoderm 77-kDa microtubule-associated protein (EMAP). The human homologue of Echinoderm microtubule-associated protein defines a novel human gene. We propose that the human EMAP is a strong candidate for the USH1a gene based on its genomic location and the proposed function of the protein.

The genomic structure of the gene defective in Usher syndrome type Ib (MYO7A)

Usher syndrome type Ib is a recessive autosomal disorder manifested by congenital deafness, vestibular dysfunction, and progressive retinal degeneration. Mutations in the human myosin VIIa gene (MYO7A) have been reported to cause Usher type Ib. Here we report the genomic organization of MYO7A. An STS content map was determined to discover the YAC clones that would cover the critical region for Usher syndrome type Ib. Three of the YACs (802A5, 966D6, and 965F10) were subcloned into cosmids and used to assemble a preliminary cosmid contig of the critical region. Part of the gene encoding human myosin VIIa was found in the preliminary cosmid contig. A cosmid, P1, PAC, and long PCR contig that contained the entire MYO7A gene was assembled. Primers were designed from the composite cDNA sequence and used to detect intron-exon junctions by directly sequencing cosmid, P1, PAC, and genomic PCR DNA. Alternatively spliced products were transcribed from the MYO7A gene: the largest transcript (7.4 kb) contains 49 exons. The MYO7A gene is relatively large, spanning approximately 120 kb of genomic DNA on chromosome 11q13.

Myosin VIIA mutation screening in 189 Usher syndrome type 1 patients

Usher syndrome type 1b (USH1B) is an autosomal recessive disorder characterized by congenital profound hearing loss, vestibular abnormalities, and retinitis pigmentosa. The disorder has recently been shown to be caused by mutations in the myosin VIIa gene (MYO7A) located on 11q14. In the current study, a panel of 189 genetically independent Usher I cases were screened for the presence of mutations in the N-terminal coding portion of the motor domain of MYO7A by heteroduplex analysis of 14 exons. Twenty-three mutations were found segregating with the disease in 20 families. Of the 23 mutations, 13 were unique, and 2 of the 13 unique mutations (Arg212His and Arg212Cys) accounted for the greatest percentage of observed mutant alleles (8/23, 31%). Six of the 13 mutations caused premature stop codons, 6 caused changes in the amino acid sequence of the myosin VIIa protein, and 1 resulted in a splicing defect. Three patients were homozygotes or compound heterozygotes for mutant alleles; these three cases were Tyr333Stop/Tyr333Stop, Arg212His-Arg302His/Arg212His-Arg302His, and IVS13nt-8c-->g/Glu450Gln. All the other USH1B mutations observed were simple heterozygotes, and it is presumed that the mutation on the other allele is present in the unscreened regions of the gene. None of the mutations reported here were observed in 96 unrelated control samples, although several polymorphisms were detected. These results add three patients to single case reported previously where mutations have been found in both alleles and raises the total number of unique mutations in MYO7A to 16.

Molecular cloning and domain structure of human myosin-VIIa, the gene product defective in Usher syndrome 1B

Myosin-VIIa is an unconventional myosin with relatively restricted expression. Cloned first from an intestinal epithelium cell line, it occurs most notably in the testis, in the receptor cells of the inner ear, and in the pigment epithelium of the retina. Defects in myosin-VIIa cause the shaker-1 phenotype in mice and Usher syndrome 1B in human, which are characterized by deafness, lack of vestibular function, and (in human) progressive retinal degeneration. Because the described cDNAs encode less than half of the protein predicted from immunoblots, we have cloned cDNAs encoding the rest of human myosin-VIIa. Two transcripts were found, one encoding the predicted 250-kDa protein and another encoding a shorter form. Both transcripts were found in highest abundance in testis, although the shorter transcript was much less abundant. Both could be detected in lymphocytes by RT-PCR. The myosin tail encoded by the long transcript includes a long repeat of approximately 460 amino acids. Each repeat contains a novel "MyTH4" domain similar to domains in three other myosins, and a domain similar to the membrane-associated portion of talin and other members of the band-4.1 family.

The construction of a yeast artificial chromosome (YAC) contig in the vicinity of the Usher syndrome type IIa (USH2A) gene in 1q41

The gene for Usher syndrome type II (USH2A), an autosomal recessive syndromic deafness, has been mapped to a region of 1q41 flanked proximally by D1S217 and distally by D1S439. Using sequence-tagged sites (STSs) within the region, a total of 21 yeast artificial chromosome (YAC) clones were isolated and ordered into a single contig that spans approximately 11.0 Mb. The order of microsatellite and STS markers in this region was established as D1S505-D1S425-DXS217-D1S556-D1S237-D1S4 74-EB1-EB2-KB6-AFM144XF2-KB1-K B4-D1S229-D1S490-D1S227-TGFbeta2-D1S439. Analysis of newly positioned polymorphic markers in recombinant individuals in two Usher syndrome type IIa families has enabled us to identify DXS474 and AFM144XF2 as two flanking markers for the Usher type IIa locus. The physical distance between the two markers is 1.0 Mb. This region is covered by eight YACs from the CEPH library: 945f7, 867g9, 762a6, 919h3, 794b8, 785h4, 848b9, and 841g2. A long-range physical map of the Usher type IIa critical region, using MluI, BssHII, NotI, EagI, and SacII, has been developed.

Differential induction of mRNAs for the glycolytic and ethanolic fermentative pathways by hypoxia and anoxia in maize seedlings

Fructose-1,6-biphosphate aldolase (ALD) and enolase (ENO) from the glycolytic pathway and pyruvate decarboxylase (PDC) and alcohol dehydrogenase 2 (ADH2) from the ethanolic fermentative pathway, are enzymes previously identified as among those synthesized selectively in O2-deficient roots of maize (Zea mays L.). The present study measured levels of transcripts representing these two pathways in 5-mm root tips, root axes (the remainder of the primary seminal root), and shoots of maize seedlings to determine how closely both pathways were co-induced and how they were modulated by changes in O2 concentration. In hypoxic seedlings with the roots in solution sparged with 5% (v/v) O2 (balance N2) and the shoots in the same gaseous atmosphere, mRNAs for Pdc1 and Adh2 in root tips both increased about 15-fold during the first 12 h, followed by a decline toward initial levels by 18 to 24h. Message levels for Ald1 and Eno1 showed only small changes during hypoxia. When expression was examined under anoxia, the extent to which all four mRNAs increased in different tissues depended on whether the seedlings had been previously acclimated to hypoxia or were anoxically shocked. The results show that although all the genes examined increased expression during hypoxia and/or anoxia, they differed in the rapidity and magnitude of the response and in the time to reach maximal message levels: there was no common pattern of change of message levels for the glycolytic or for the fermantative enzymes.

Concerted proton-electron transfer between ascorbic acid and cytochrome b561

Ascorbic acid is an essential reductant in biology but its reducing power is paradoxical. At physiological pH the predominant form of ascorbate (the monoanion) is a poor electron donor because it oxidizes to the energetically unfavorable neutral free radical. The ascorbate dianion forms the relatively stable semidehydroascorbate radical anion and is a powerful electron donor but its concentration at neutral pH is insufficient to produce the reaction rates observed. For example, ascorbate rapidly reduces cytochrome b561 from adrenal medullary chromaffin vesicles. This fast reaction rate may be rationalized by a mechanism involving concerted proton-electron transfer rather than electron transfer alone. This would permit reduction of the cytochrome by the abundant ascorbate monoanion but would circumvent formation of unfavorable intermediates. This may be a general mechanism of biological ascorbic acid utilization: enzymes using ascorbic acid may react with the ascorbate monoanion via concerted proton-electron transfer.

Vitamins C and E donate single hydrogen atoms in vivo

The antioxidant vitamins, C and E, eliminate cytotoxic free radicals by redox cycling. Energetic and kinetic considerations suggest that cycling of vitamin C and vitamin E between their reduced and free radical forms occurs via the transfer of single hydrogen atoms rather than via separate electron transfer and protonation reactions. This may enable these vitamins to reduce many of the damaging free radicals commonly encountered by biological systems while minimizing the reduction of molecular oxygen to superoxide.

Reaction of ascorbic acid with cytochrome b561. Concerted electron and proton transfer

Rate constants for reduction of cytochrome b561 by internal ascorbate (k0A) and oxidation by external ferricyanide (k1F) were determined as a function of pH from rates of steady-state electron transfer across chromaffin-vesicle membranes. The pH dependence of electron transfer from cytochrome b561 to ferricyanide (k1F) may be attributed to the pH dependence of the membrane surface potential. The rate constant for reduction by internal ascorbate (k0A), like the previously measured rate constant for reduction by external ascorbate (k-1A), is not very pH-dependent and is not consistent with reduction of cytochrome b561 by the ascorbate dianion. The rate at which ascorbate reduces cytochrome b561 is orders of magnitude faster than the rate at which it reduces cytochrome c, despite the fact that midpoint reduction potentials favor reduction of cytochrome c. Moreover, the rate constant for oxidation of cytochrome b561 by ferricyanide (k1F) is smaller than the previously measured rate constant for oxidation by semidehydroascorbate, despite the fact that ferricyanide has a higher midpoint reduction potential. These results may be reconciled by a mechanism in which electron transfer between cytochrome b561 and ascorbate/semidehydroascorbate is accelerated by concerted transfer of a proton. This may be a general property of biologically significant electron transfer reactions of ascorbic acid.

Rate of electron transfer between cytochrome b561 and extravesicular ascorbic acid

Cytochrome b561 transfers electrons across secretory vesicle membranes in order to regenerate intravesicular ascorbic acid. To show that cytosolic ascorbic acid is kinetically competent to function as the external electron donor for this process, electron transfer rates between cytochrome b561 in adrenal medullary chromaffin vesicle membranes and external ascorbate/semidehydroascorbate were measured. The reduction of cytochrome b561 by external ascorbate may be measured by a stopped-flow method. The rate constant is 450 (+/- 190) M-1 s-1 at pH 7.0 and increases slightly with pH. The rate of oxidation of cytochrome b561 by external semidehydroascorbate may be deduced from rates of steady-state electron flow. The rate constant is 1.2 (+/- 0.5) x 10(6) M-1 s-1 at pH 7.0 and decreases strongly with pH. The ratio of the rate constants is consistent with the relative midpoint reduction potentials of cytochrome b561 and ascorbate/semidehydroascorbate. These results suggest that cytosolic ascorbate will reduce cytochrome b561 rapidly enough to keep the cytochrome in a mostly reduced state and maintain the necessary electron flux into vesicles. This supports the concept that cytochrome b561 shuttles electrons from cytosolic ascorbate to intravesicular semidehydroascorbate, thereby ensuring a constant source of reducing equivalents for intravesicular monooxygenases.

A kinetic analysis of electron transport across chromaffin vesicle membranes

Some types of secretory vesicles, such as the chromaffin vesicles of the adrenal medulla, have cytochrome b561 which is believed to mediate the transfer of electrons across the vesicle membrane. To characterize the kinetics of this process, we have examined the rate of electron transfer from ascorbate trapped within chromaffin vesicle ghosts to external ferricyanide. The rate of ferricyanide reduction saturates at high ferricyanide concentrations. The reciprocal of the rate is linearly related to the reciprocal of the ferricyanide concentration. The internal ascorbate concentration affects the y intercept of this double-reciprocal plot but not the slope. These observations and theoretical considerations indicate that the slope is associated with a rate constant k1 for the oxidation of cytochrome b561 by ferricyanide. The intercept is associated with a rate constant k0 for the reduction of cytochrome b561 by internal ascorbate. From k0 and standard reduction potentials, the rate constant k-0 for the reduction of internal semidehydroascorbate by cytochrome b561 can be calculated. Under conditions prevailing in vivo, this rate of semidehydroascorbate reduction appears to be much faster than the expected rate of semidehydroascorbate disproportionation. This supports the hypothesis that cytochrome b561 functions in vivo to reduce intravesicular semidehydroascorbate thereby maintaining intravesicular ascorbic acid.

Mechanism of ascorbic acid regeneration mediated by cytochrome b561

In summary, ascorbic acid serves as a one-electron donor for dopamine beta-hydroxylase in chromaffin vesicles and probably for peptide amidating monooxygenase in neurohypophyseal secretory vesicles. It appears that the semidehydroascorbate that is produced is reduced by cytochrome b561 to regenerate intravesicular ascorbate. Cytochrome b561, a transmembrane protein, is reduced in turn by an extravesicular electron donor, probably cytosolic ascorbic acid. It will be interesting to see whether other ascorbate-requiring enzymes in other organelles use a similar ascorbate-regenerating system to provide an intravesicular supply of reducing equivalents.

Differential induction of heat shock, SOS, and oxidation stress regulons and accumulation of nucleotides in Escherichia coli

Heat and various inhibitory chemicals were tested in Escherichia coli for the ability to cause accumulation of adenylylated nucleotides and to induce proteins of the heat shock (htpR-controlled), the oxidation stress (oxyR-controlled), and the SOS (lexA-controlled) regulons. Under the conditions used, heat and ethanol initiated solely a heat shock response, hydrogen peroxide and 6-amino-7-chloro-5,8-dioxoquinoline (ACDQ) induced primarily an oxidation stress response and secondarily an SOS response, nalidixic acid and puromycin induced primarily an SOS and secondarily a heat shock response, isoleucine restriction induced a poor heat shock response, and CdCl2 strongly induced all three stress responses. ACDQ, CdCl2, and H2O2 each stimulated the synthesis of approximately 35 proteins by factors of 5- to 50-fold, and the heat shock, oxidation stress, and SOS regulons constituted a minor fraction of the overall cellular response. The pattern of accumulation of adenylylated nucleotides during these treatments was inconsistent with a simple role for these nucleotides as alarmones sufficient for triggering the heat shock response, but was consistent with a role in the oxyR-mediated response.

The complete amino Acid sequence for the anaerobically induced aldolase from maize derived from cDNA clones

A cDNA library was synthesized from maize anaerobic root mRNA and screened with cDNA specific to the anaerobically induced Zea mays cytoplasmic aldolase. At least 1% of the cDNA of the library corresponded to maize cytoplasmic aldolase. The sequence of four overlapping cDNA clones encoded a protein of molecular weight 38,611 homologous to aldolase. These cDNAs were polymorphic at three bases and one of these cDNAs had a different, shorter 3'-untranslated region. No known eukaryotic poly(A) addition site was detected. The derived amino acid sequences of maize was compared to the sequence of aldolase of trypanosome, Drosophila, and two mammalian isozymes, A and B. Of these, maize cytoplasmic aldolase was found to have the highest homology (55%) with rabbit aldolase A.

Cytochrome b561 spectral changes associated with electron transfer in chromaffin-vesicle ghosts

The involvement of cytochrome b561, an integral membrane protein, in electron transfer across chromaffin-vesicle membranes is confirmed by changes in its redox state observed as changes in the absorption spectrum occurring during electron transfer. In ascorbate-loaded chromaffin-vesicle ghosts, cytochrome b561 is nearly completely reduced and exhibits an absorption maximum at 561 nm. When ferricyanide is added to a suspension of these ghosts, the cytochrome becomes oxidized as indicated by the disappearance of the 561 nm absorption. If a small amount of ferricyanide is added, it becomes completely reduced by electron transfer from intravesicular ascorbate. When this happens, cytochrome b561 returns to its reduced state. If an excess of ferricyanide is added, the intravesicular ascorbate becomes exhausted and the cytochrome b561 remains oxidized. The spectrum of these absorbance changes correlates with the difference spectrum (reduced-oxidized) of cytochrome b561. Cytochrome b561 becomes transiently oxidized when ascorbate oxidase is added to a suspension of ascorbate-loaded ghosts. Since dehydroascorbate does not oxidize cytochrome b561, it is likely that oxidation is caused by semidehydroascorbate generated by ascorbate oxidase acting on free ascorbate. This suggests that cytochrome b561 can reduce semidehydroascorbate and supports the hypothesis that the function of cytochrome b561 in vivo is to transfer electrons into chromaffin vesicles to reduce internal semidehydroascorbate to ascorbate.

Coordinate induction of alcohol dehydrogenase 1, aldolase, and other anaerobic RNAs in maize

Anaerobiosis results in the selective synthesis of a particular set of polypeptides in the maize root including the two alcohol dehydrogenases (Sachs, M. M., Freeling, M., and Okimoto, R. (1980) Cell 20, 761-768), pyruvate decarboxylase (Wignarajah, K., and Greenway, H. (1976) New Phytol. 77, 575-584; Laszlo, A., and St. Lawrence, P. (1983) Mol. Gen. Genet. 192, 110-117), glucose phosphate isomerase (Kelley, P. M., and Freeling, M. (1984) J. Biol. Chem. 259, 673-677) and aldolase (Kelley, P. M., and Freeling, M. (1984) J. Biol. Chem. 259, 14180-14183). This report describes the identification and characterization of cDNA clones to five different mRNA species induced upon anaerobic shock. Immunoprecipitation of hybrid-selected translation polypeptides has determined the identity of the cDNA clone for fructose-1,6-diphosphate aldolase mRNA. Quantitative hybridization analysis of anaerobic mRNAs using the cDNA clones has shown that there is not a simultaneous accumulation of anaerobic mRNAs. Upon reintroduction of air, the anaerobic mRNAs disappear rapidly and at approximately the same rate. A translocation line that generates progeny that contain 1, 2, and 3 doses of the long arm of chromosome one (1L) allowed us to test for clustering of the anaerobic genes; two of the anaerobic genes tested do not reside with Adh 1 and Phi 1 on the long arm of chromosome 1.

Anaerobic expression of maize fructose-1,6-diphosphate aldolase

The anaerobic proteins of maize are a set of 10 major and 10 minor polypeptides selectively synthesized in anaerobic seedling roots. 1) Anaerobiosis resulted in the selected labeling of a protein which bound to Blue Sepharose and was eluted by fructose 1,6-diphosphate. 2) This protein elicited antiserum which recognized a single protein with molecular weight of approximately 40,000. 3) By Western blot analysis, this antiserum recognized a maize fructose-1,6-diphosphate aldolase purified to homogeneity. We show that two major anaerobic proteins of maize, ANP35.5 and ANP33A, correspond to a cytoplasmic fructose-1,6-diphosphate aldolase.