Arabinoxylan degradation by fungi: characterization of the arabinoxylan-arabinofuranohydrolase encoding genes from Aspergillus niger and Aspergillus tubingensis

The genes encoding the enzyme arabinoxylan arabinofuranohydrolase, which releases L-arabinose from arabinoxylan, have been cloned from the closely related fungi Aspergillus niger and Aspergillus tubingensis and were shown to be functional in A. niger. Integration of multiple copies in the genome resulted in over-expression of the enzymes. The arabinofuranohydrolases encoded comprise 332 amino acids and have 94% amino acid identity. Their primary structure is not related to those of other alpha-L-arabinofuranosidases, except for a low similarity with XYLC, a bacterial alpha-L-arabinofuranosidase from Pseudomonas fluorescens which acts on oat spelt xylan. The axhA expression pattern in A. niger differed from that of abfB, since it was strongly induced by birchwood xylan and much less by L-arabitol or L-arabinose. Furthermore, Northern analysis revealed that axhA expression was de repressed in creAd mutants and carbon catabolite repressed by D-glucose.

Isolation of expressed sequence tags of Agaricus bisporus and their assignment to chromosomes

The genome of the cultivated basidiomycete Agaricus bisporus Horst U1 and of its homokaryotic parents has been characterized by using an optimized method of pulsed-field gel electrophoresis. Expressed sequence tags obtained as expressed cDNAs from a primordial tissue-derived cDNA library and a number of previously isolated genes were used to identify the individual chromosomes of the parental lines of Horst U1. The genome consists of 13 chromosomes, and its total size is 31 Mb. For those chromosomes that could not be resolved by contour-clamped homogeneous electric field electrophoresis, the segregation of marker genes was studied in a set of 86 homokaryotic offspring of Horst U1. At least two markers were assigned to each individual chromosome. In this way all individual chromosomes were unequivocally identified. The large size difference observed between the homologous chromosomes IX, harboring the rDNA repeat, was shown to be largely due to a higher copy number of rDNA in parental strain H97 than in parental strain H39.

Identification of regulatory mutants of Aspergillus aculeatus affected in rhamnogalacturonan hydrolase expression

Rhamnogalacturonan hydrolase expression in A. aculeatus can be induced by pectin, but also by a combination of two constituent monosaccharides of pectin, rhamnose and galacturonic acid. The rhgA promoter was fused to the A. niger glucose oxidase coding sequence and a single copy of the hybrid gene was integrated at the rhgA locus in the genome of A. aculeatus. The gene product was subsequently used as reporter in a screening assay for the selection of rhamnogalacturonan hydrolase-overproducing mutant strains. At least four of the mutations were recessive and could be assigned to different loci. One mutation (rgr25) showed linkage with the rhgA locus. Inducible rhamnogalacturonan hydrolase expression levels of about 5-10 times that in the wild-type were found in the mutants rgr48, rgr25 and rgr34 after growth on a combination of rhamnose and galacturonic acid with or without fructose as a carbon source. In mutant rgr48 elevated levels of rhgA transcription were found.

Identification, isolation and sequence of the Aspergillus nidulans xlnC gene encoding the 34-kDa xylanase

The xlnC gene encoding the 34-kDa xylanase (X34) of Aspergillus nidulans (An) has been cloned and sequenced, as has its corresponding cDNA. xlnC contains nine introns and shows considerable similarity to the xynA and xylP xylanase-encoding genes of A. kawachii (Ak) and Penicillium chrysogenum (Pc), respectively. Analysis of xylanase production in An multicopy transformants showed elevated levels of X34 and increased total xylanase activity, but no elevated production of other xylanases. Northern analysis demonstrated transcriptional induction by xylan and repression by glucose.

Cloning and Biochemical Characterisation of an Aspergillus Niger Glucokinase. Evidence for the Presence of Separate Glucokinase and Hexokinase Enzymes

The Aspergillus niger glucokinase gene glkA has been cloned using a probe generated by polymerase chain reaction with degenerate oligonucleotides. The DNA sequence of the gene was determined, and the deduced amino acid sequence shows significant similarity to other eukaryotic hexokinase and glucokinase proteins, in particular to the Saccharomyces cerevisiae glucokinase protein. The encoded protein was purified from a multicopy glkA transformant, and extensively characterised. The protein has a molecular mass of 54536 Da and a pI of 5.2. The enzyme has high affinity for glucose (K(m) 0.063 mM at pH 7.5) and a relatively low affinity for fructose (K(m) 120 mM at pH 7.5), and in vivo fructose phosphorylation by glucokinase is consequently negligible. The configurations at C1 and C4 of the substrate appear to be essential for substrate specificity. The A. niger glucokinase shows non-competitive inhibition by ADP towards ATP and uncompetitive inhibition by ADP towards glucose. The kcal (turnover number) decreases rapidly below pH 7.5 (56% at pH 7.0 and 17% at pH 6.5) and this may have important implications for the in vivo regulation of activity. In addition, proof is provided for the presence of a second hexosephosphorylating enzyme in A. niger. This enzyme is probably a hexokinase, since unlike glucokinase, this activity is inhibited by trehalose 6-phosphate.

Primary structure and characterization of an exopolygalacturonase from Aspergillus tubingensis

From the culture fluid of the hyphal fungus Aspergillus tubingensis, an exopolygalacturonase with a molecular mass of 78 kDa, an isoelectric point in the pH-range 3.7-4.4 and a pH optimum of 4.2 was purified. The enzyme has been characterized as an exopolygalacturonase [poly(1,4-alpha-D-galacturonide)galacturonohydrolase] that cleaves monomer units from the non-reducing end of the substrate molecule. K(m) and Vmax for polygalacturonic acid hydrolysis were 3.2 mg ml-1 and 3.1 mg ml-1 and 255 U mg-1 and 262 U mg-1 for the wild-type and recombinant enzymes, respectively. The kinetic data of exopolygalacturonase on oligogalacturonates of different degree of polymerization (2-7) were interpreted in terms of a subsite model to obtain more insight into catalysis and substrate binding. On oligogalacturonates of different degrees of polymerization (2-7), the Michaelis constant (K(m)) decreased with increasing chain length (n). The Vmax value increased with chain length up to n = 4, then reached a plateau value. The enzyme was competitively inhibited by galacturonic acid (Ki = 0.3 mM) as well as by reduced digalacturonate (Ki = 0.4 mM). The exopolygalacturonase gene (pgaX) was cloned by reverse genetics and shows only 13% overall amino acid sequence identity with A. niger endopolygalacturonases. The exopolygalacturonase is most related to plant polygalacturonases. Only four small stretches of amino acids are conserved between all known endogalacturonases and exopolygalacturonases. Expression of the pgaX gene is inducible with galacturonic acid and is subject to catabolite repression. A fusion between the promoter of the A. niger glycolytic gene encoding pyruvate kinase and the pgaX-coding region was used to achieve high level production of exopolygalacturonase under conditions where no endopolygalacturonases were produced.

Regulation of acid phosphatases in an Aspergillus niger pacC disruption strain

An Aspergillus niger strain has been constructed in which the pH-dependent regulatory gene, pacC, was disrupted. The pacC gene of A. niger, like that of A. nidulans, is involved in the regulation of acid phosphatase expression. Disruptants were identified by a reduction in acid phosphatase staining of colonies. Southern analysis demonstrated integration of the disruption plasmid at the pacC locus and Northern analysis showed that the disruption strain produced a truncated pacC mRNA of 2.2 kb (as compared to 2.8 kb in the wild type). The strain carrying the pacC disruption was used to assign the pacC gene to linkage group IV; this was confirmed by CHEF electrophoresis and Southern analysis. This strain further allowed us to determine which extracellular enzyme and transport systems are under the control of pacC in A. niger. Expression of the A. niger pacC wild-type gene and the truncated pacC gene showed that, in contrast to the auto-regulated wild-type expression, which was elevated only at alkaline pH, the truncated pacC gene was deregulated, as high-level expression occurred regardless of the pH of the culture medium. Analysis of the phosphatase spectrum by isoelectric focussing and enzyme activity staining both in the wild-type and the pacC disruptant showed that at least three acid phosphatases are regulated by the pacC. For the single alkaline phosphatase no pH regulation was observed.

Characterisation of the Aspergillus nidulans frA1 mutant: hexose phosphorylation and apparent lack of involvement of hexokinase in glucose repression

Hexose phosphorylation was studied in Aspergillus nidulans wild-type and in a fructose non-utilising mutant (frA). The data indicate the presence of at least one hexokinase and one glucokinase in wild-type A. nidulans, while the frA1 mutant lacks hexokinase activity. The A. nidulans gene encoding hexokinase was isolated by complementation of the frA1 mutation. The absence of hexokinase activity in the frA1 mutant did not interfere with glucose repression of the enzymes involved in alcohol and L-arabinose catabolism. This suggest that, unlike the situation in yeast where mutation of hexokinase PII abolishes glucose repression, the A. nidulans hexokinase might not be involved in glucose repression.

Molecular cloning and expression in Saccharomyces cerevisiae of two Aspergillus nidulans xylanase genes

Two Aspergillus nidulans genes, xlnA and xlnB, encoding the X22 and X24 xylanases from this fungus, respectively, have been cloned and sequenced. Their cDNAs have been expressed in a laboratory Saccharomyces cerevisiae strain under the control of a constitutive yeast promoter, resulting in the construction of recombinant xylanolytic yeast strains.

The Agaricus bisporus hypA gene encodes a hydrophobin and specifically accumulates in peel tissue of mushroom caps during fruit body development

Differential screening of a cDNA library was used to clone genes that are specifically expressed during mushroom development in the basidiomycete Agaricus bisporus. One of the isolated genes encodes a polypeptide of 112 amino acid residues and belongs to the fungal gene family encoding hydrophobins. This gene, hypA, has the characteristic pattern of eight cysteine residues at conserved positions and a hydrophobicity pattern that is very similar to class I hydrophobins. Elucidation of the genomic structure of hypA led to the identification of a second copy, hypC, located downstream of hypA. Although at a much lower level, hypC is like hypA specifically expressed on fruit bodies. The hypA mRNA level is transiently increased ten days after fruit body induction and expression appears to be associated with rapid expansion of the mushroom caps. In mushroom caps, very high concentrations of hypA messengers were found in the (outer) peel tissue, where they accumulate to more than 60% of the total mRNA mass. The corresponding protein with a molecular mass of 8 to 9 kDa was purified from this peel tissue and was identified by N-terminal sequencing. Our results suggest that HYPA forms a protective hydrophobic layer instrumental in cap formation.

Expression of an Erwinia pectate lyase in three species of Aspergillus

Transgenic filamentous fungi of the species Aspergillus niger, A. nidulans and A. awamori expressing and secreting Erwinia carotovora subsp. atroseptica pectate lyase 3 (PL3) were generated. Correct processing of the pre-enzyme was achieved using the A. niger pectin lyase A (PEL A) signal peptide. With the prepro-peptide of A. niger polygalacturonase II, secreted enzymes still possessed the 6- aa pro-sequence, indicating the importance of the conformation of the precursor protein for correct cleavage of the signal sequence. PL3 expression was markedly increased in media optimized for limited protease activity, and reached 0.4, 0.8 and 2.0 mg/l for expression in A. niger, A. awamori and A. nidulans, respectively. Glycans attached to the PL3 enzymes exhibited species-specific differences, and an increase of molecular mass coincided with reduced specific activities of the enzymes.

Inversion of configuration during hydrolysis of alpha-1,4-galacturonidic linkage by three Aspergillus polygalacturonases

Endopolygalacturonases I and II (PGI and PGII) of Aspergillus niger and an exopolygalacturonase (ExoPG) of A. tubingensis were investigated to reveal the stereochemistry of their hydrolytic action. Reduced pentagalacturonic acid (pentaGalU-ol) and reduced trigalacturonic acid (triGalU-ol) were used as non-reducing substrates for the enzymes. The configuration of the reducing ends in the products formed in D2O reaction mixtures was followed by 1H-NMR spectroscopy. It has been unambiguously established that primary cleavage of pentaGalU-ol by both PGI and PGII leads to diGalU-ol and the beta-anomer of triGalUA. The primary products of hydrolysis of triGalUA-ol by ExoPG were diGal-ol and the beta-anomer of GalUA. Thus, all three Aspergillus polygalacturonases belong to the so-called inverting glycanases, i.e. they utilize the single displacement mechanism of hydrolysis of the glycosidic linkage.

Identification, cloning and analysis of the Aspergillus niger gene pacC, a wide domain regulatory gene responsive to ambient pH

A wide domain regulatory gene implicated in modulating gene expression in response to ambient pH has been cloned and sequenced from the industrially useful filamentous fungus Aspergillus niger. This gene, pacC, is able to restore a pacC+ phenotype to A. nidulans pacCc11 and pacCc14 mutants with respect to extent of conidiation, conidial pigment intensity and acid phosphatase regulation. The pacC gene of A. niger comprises three exons, encodes a three-zinc-finger protein of 677 amino acids, and shows pH-dependent regulation of expression: mRNA levels are elevated under alkaline conditions and considerably reduced under acidic conditions. The occurrence of PacC consensus binding targets within the sequences upstream of pacC may indicate autoregulation.

Nucleotide sequence and expression of the gene encoding NADP+- dependent glutamate dehydrogenase (gdhA) from Agaricus bisporus

The gene encoding NADP+-dependent glutamate dehydrogenase (gdhA) was isolated from an Agaricus bisporus recombinant phage lambda library. The deduced amino acid sequence would specify a 457-amino acid protein that is highly homologous in sequence to those derived from previously isolated and characterized genes coding for microbial NADP+-GDH. The open reading frame is interrupted by six introns. None of the introns is located at either one of the positions of the two introns conserved in the corresponding open reading frames of the ascomycete fungi Aspergillus nidulans and Neurospora crassa. Northern analysis suggests that the A. bisporus gdhA gene is transcriptionally regulated and that, unlike the case in ascomycetes, transcription of this gene is repressed upon the addition of ammonium to the culture.

Cloning and characterisation of genes (pkc1 and pkcA) encoding protein kinase C homologues from Trichoderma reesei and Aspergillus niger

Oligonucleotides, designed on the basis of conserved flanking amino acid sequence segments within the catalytic domain of eukaryotic protein kinase C (PKC) proteins, were used as primers for polymerase chain reactions to amplify a 427-bp chromosomal DNA fragment from the filamentous fungus Trichoderma reesei. This fragment was then used to isolate genes encoding PKC homologues of T. reesei and Aspergillus niger (pkc1 and pkcA, respectively). The genes contain six (T. reesei) and eight (A. niger) introns, which exhibit notable conservation in position with those found in the corresponding Schizosaccharomyces pombe pkc1+ and Drosophila melanogaster dPKC53Ebr genes. A single 4.2-kb transcript was detected in Northern analyses. The deduced PKC1 (T.reesei, 126 kDa) and PKCA (A. niger, 122 kDa) amino acid sequences reveal domains homologous to the C1 and C3/C4 domains of PKC-related proteins, but lack typical Ca(2+)-binding (C2) domains. Both contain a large, extended N-terminus, which shares a high degree of similarity with the corresponding regions of Saccharomyces cerevisiae PKC1 and S. pombe pkc1+ and pkc2+ proteins, but which is not present in PKCs of Dictyostelium or higher eukaryotes. This extended region can be divided into three subdomains; the N-terminal one contains a hydrophobic helix-turn-helix motif, whereas the C-terminal one contains potential targets for proteolytic processing. A polyclonal antiserum raised against the pseudosubstrate-binding domain of PKC1 recognizes in T. reesei a 115-120 kDa protein in Western blots. Expression of pkc1 cDNA in insect cells directs the synthesis of a PKC1 protein of similar size. The T. reesei PKC1 protein was partially purified and some of its properties examined: it is stimulated about twofold by phospholipids or phorbol esters but is not stimulated by Ca2+. We conclude that these PKC proteins from filamentous fungi represent the Ca(2+)-insensitive fungal homologues of the nPKC family.

Peripherin: a novel marker for the immunohistochemical study of malformations of the enteric nervous system

Pheripherin is a 57-kD type III intermediate filament that is a specific marker for peripheral neruons, including enteric ganglion cells (GCs). Hence antibodies to peripherin may be used to demonstrate abnormalities of the enteric nervous system (ENS). Serial longitudinal histologic sections of formalin-fixed paraffin-embedded colons from 15 patients were immunostained for peripherin, neuron-specific enolase (NSE), neurofilaments, S-100, and synaptophysin. Ten patients had variable degrees of colonic aganglionosis (Hirschsprung's disease), three were premature in infants, and two were controls. Peripherin labeling yielded the highest number of recognizable GCs. Overall, 56%, 78%, and 80% of the peripherin-positive GCs in the myenteric plexus were identified by staining for neurofilaments, NSE, and S-100, respectively. Intramucosal GCs were detected in 4 of 10 cases of Hirschspring's disease (HD), none of which had been evident by routine histology. The other neuronal markers were less specific for intramucosal GCs than peripherin, because they also added enterochromaffin cells. Peripherin immunohistochemistry also allowed exact quantification of GC density expressed as GCs/mm colon, which is important for the diagnosis of HD-related disorders. In three cases of HD the GC density of the transition zone was markedly elevated compared with more proximal ganglionic bowel segments, consistent with neuronal intestinal dysplasia type B, and two cases of HD showed low GC density within the transition zone. Hence peripherin immunolabeling may prove to be a valuable aid in the diagnosis and classification of congenital malformations of the ENS.

Polyol pools in Aspergillus niger

The accumulation and excretion of polyols was investigated under various growth conditions and at different stages of the life cycle of Aspergillus niger. Glycerol was found to be the major solute in osmotic adjustment of the hyphae. Conidiospores contain large amounts of mannitol, which are rapidly metabolized during early germination, leading to the accumulation of glycerol. Glycerol is the major polyol in young mycelium, whereas in older mycelium mannitol and erythritol predominate. In all experiments, polyols were also excreted. The mechanism and function of this process is unknown, but it might be a way to control the levels of the intracellular polyol pools. Polyols are rapidly taken up again upon starvation. In a glycerol kinase mutant the synthesis of glycerol is unaffected but the excreted level of the polyol is higher. This glycerol is taken up again upon starvation, and accumulates intracellularly as it can not be metabolized further.

Cloning, sequence and expression of the gene coding for rhamnogalacturonase of Aspergillus aculeatus; a novel pectinolytic enzyme

Rhamnogalacturonase was purified from culture filtrate of Aspergillus aculeatus after growth in medium with sugar-beet pulp as carbon source. Purified protein was used to raise antibodies in mice and with the antiserum obtained a gene coding for rhamnogalacturonase (rhgA) was isolated from a lambda cDNA expression library. The cloned rhgA gene has an open-reading frame of 1320 base pairs encoding a protein of 440 amino acids with a predicted molecular mass of 45 962 Da. The protein contains a potential signal peptidase cleavage site behind Gly-18 and three potential sites for N-glycosylation. Limited homology with A. niger polygalacturonase amino acid sequences is found. A genomic clone of rhgA was isolated from a recombinant phage lambda genomic library. Comparison of the genomic and cDNA sequences revealed that the coding region of the gene is interrupted by three introns. Furthermore, amino acid sequences of four different peptides, derived from purified A. aculeatus rhamnogalacturonase, were also found in the deduced amino acid sequence of rhgA. A. aculeatus strains overexpressing rhamnogalacturonase were obtained by cotransformation using either the A. niger pyrA gene or the A. aculeatus pyrA gene as selection marker. For expression of rhamnogalacturonase in A. awamori the A. awamori pyrA gene was used as selection marker. Degradation patterns of modified hairy regions, determined by HPLC, show the recombinant rhamnogalacturonase to be active, and the enzyme was found to have a positive effect in the apple hot-mash liquefaction process.

Complete degradation of tetrachloroethene by combining anaerobic dechlorinating and aerobic methanotrophic enrichment cultures

Degradation of tetrachloroethene (perchloroethylene, PCE) was investigated by combining the metabolic abilities of anaerobic bacteria, capable of reductive dechlorination of PCE, with those of aerobic methanotrophic bacteria, capable of co-metabolic degradation of the less-chlorinated ethenes formed by reductive dechlorination of PCE. Anaerobic communities reductively dechlorinating PCE, trichloroethene (TCE) and dichloroethenes were enriched from various sources. The maximum rates of dechlorination observed for various chloroethenes in these batch enrichments were: PCE to TCE (341 mumol l-1 day-1), TCE to cis-dichloroethene (159 mumol l-1 day-1), cis-dichloroethene to chloroethene (99 mumol l-1 day-1) and trans-dichloroethene to chloroethene (22 mumol l-1 day-1). A mixture of these enrichments was inoculated into an anoxic fixed-bed upflow column. In this column PCE was converted mainly into cis-1,2-dichloroethene, small amounts of TCE and chloroethene, and chloride. Enrichments of aerobic methanotrophic bacteria were grown in an oxic fixed-bed downflow column. Less-chlorinated ethenes, formed in the anoxic column, were further metabolized in this oxic methanotrophic column. On the basis of analysis of chloride production and the disappearance of chlorinated ethenes it was demonstrated that complete degradation of PCE was possible by combining these two columns. Operation of the two-column system under various process conditions indicated that the sensitivity of the methanotrophic bacteria to chlorinated intermediates represented the bottle-neck in the sequential anoxic/oxic degradation process of PCE.

An extreme creA mutation in Aspergillus nidulans has severe effects on D-glucose utilization

Aspergillus nidulans wild-type and the extreme carbon catabolite derepressed mutant creAd-30 were characterized with respect to enzyme activities, metabolite concentrations and polyol pools all related to glycolysis, after growth on D-glucose. In the creAd-30 strain the enzymes hexokinase and fructose-6-phosphate reductase showed a two- and threefold increase in activity, respectively, whereas phosphofructokinase and pyruvate kinase activity decreased two- and threefold, respectively, in comparison with the wild-type strain. The most notable changes in metabolite concentrations were that fructose 2,6-bisphosphate and fructose 1,6-bisphosphate showed a 2.5-fold increase, whereas both pyruvate and citrate decreased in the creAd-30. Striking differences were found for the polyol concentrations measured for the two strains tested. Intracellular glycerol and arabitol concentrations were 10-fold higher and erythritol fivefold higher in creAd-30, whereas intracellular trehalose and mannitol were both decreased. The total internal polyol concentration appears to be constant at approximately 700 mumol (g dry wt)-1. All polyols were also detected in high amounts in the culture filtrate of the creAd-30 mutant strain but no extracellular trehalose was found. The overall production of polyols in this strain was therefore much higher than in the wild-type. The high level of polyols produced and the changes in metabolite concentrations in the creAd-30 strain suggest that the differences in enzyme activities result in an altered flow through glycolysis leading to a more rapid formation of polyols which are subsequently secreted.

New protease mutants in Aspergillus niger result in strongly reduced in vitro degradation of target proteins; genetical and biochemical characterization of seven complementation groups

Several mutants of Aspergillus niger, deficient in extracellular protease expression, have been isolated and characterized both genetically and biochemically. The mutant strains, obtained after in vivo UV-mutagenesis of conidiospores and selected by haloscreening on a new dual-substrate plate assay, belong to at least seven different complementation groups. These seven prt loci were assigned to linkage groups using master strains with marked chromosomes. One prt locus (prtC) could be assigned to linkage group I, three (prtB, prtE and prtG) to linkage group III, one (prtF) to linkage group V and the two remaining loci (prtA and prtD) to linkage group VIII. Extracellular proteolytic activities varied from 2 to 3% up to 80% of the protease activity of the parental strain. Assigning the different prt mutants to structural or regulatory genes is difficult since only one structural gene, pepA, has been mapped unambiguously on linkage group I but is not identical to prtC. All prt mutants except for prtC are likely to be regulatory mutants or else belong to a proteolytic cascade because residual activities showed that more proteolytic activities were affected simultaneously. Double mutants were constructed both by recombination and by a second round of mutagenesis. In both cases mutants with further reduced extracellular proteolytic activities were isolated. A sensitive in vitro degradation assay, based on the homologous pectin lyase B (PELB) protein to analyze proteolytic degradation in A. niger, was developed and used to show extremely reduced proteolytic PELB degradation in the culture media of some of these mutants.

Turnover of the D1 protein and of Photosystem II in a Synechocystis 6803 mutant lacking Tyrz

The Photosystem II reaction center is rapidly inactivated by light, particularly at higher light intensity. One of the possible factors causing this phenomenon is the oxidized primary donor, P680(+), which may be harmful to Photosystem II because of its highly oxidizing nature. However, no direct evidence specificially implicating P680(+) in photoinhibition has been obtained yet. To investigate whether P680(+) is harmful to Photosystem II, turnover of the D1 protein and of the Photosystem II reaction center complex were measured in vivo in a mutant of the cyanobacterium Synechocystis sp. PCC 6803, in which the physiological donor to P680(+), Tyrz, was genetically deleted. In this mutant, D1 degradation in the light is an order of magnitude faster than in wild type. The most straightforward explanation of this phenomenon is that accumulation of P680(+) leads to an increased rate of turnover of the Photosystem II reaction center complex, which is compatible with the hypothesis of destructive oxidation by P680(+) that is damaging to the Photosystem II complex.