The molecular features of chromosome pairing at meiosis: the polyploid challenge using wheat as a reference.

During meiosis, chromosome numbers are halved, leading to haploid gametes, a process that is crucial for the maintenance of a stable genome through successive generations. The process for the accurate segregation of the homologues starts in pre-meiosis as each homologue is replicated and the respective products are held together as two sister chromatids via specific cohesion proteins. At the start of meiosis, each chromosome must recognise its homologue from amongst all the chromosomes present in the nucleus and then associate or pair with that homologue. This process of homologue recognition in meiosis is more complicated in polyploids because of the greater number of related chromosomes. Despite the presence of these related chromosomes, for polyploids such as wheat to produce viable gametes, they must behave as diploids during meiosis with only true homologues pairing. In this review, the relationship between the Ph1 cyclin-dependent kinase (CDK)-like genes in wheat and the CDK2 genes in mammals and their involvement in controlling this process at meiosis is examined.

Structural and functional relationship between the Ph1 locus protein 5B2 in wheat and CDK2 in mammals.

The Ph1 locus in hexaploid wheat is responsible for restricting chromosome pairing at meiosis to true homologues by suppressing homoeologous pairing. Based on detailed modelling studies and predicted ability to form complexes with cyclin-A and cyclin-dependent kinase inhibitor such as p27, Triticum aestivum-5B2 (( Ta ) 5B2) is suggested to be a wheat analogue of human CDK2 enzyme. A blast analysis of the protein data bank using the amino acid sequence of the protein expressed by the 5B2 copy of the cdk-like cluster of genes at the Ph1 locus (( Ta ) 5B2) identified humans CDK2 as a top hit. In this analysis, the canonical cyclin binding motif PSTAIRE of CDK2 is replaced by a novel DARTLRE motif and Thr160 residue, phosphorylation of which is required for positive regulation of CDK2, is replaced by a tyrosine (Tyr174) in ( Ta ) 5B2. Despite these differences, detailed analyses show that all residues known to be important for cyclin binding are either fully conserved or whenever there is alteration in ( Ta ) 5B2, a corresponding but comparable alteration is also observed in plant cyclins notably cyclin-A of Arabidopsis thaliana. Moreover, the Thr160/Tyr174 substitution is also accommodated by suitable alterations in the 3D space around Tyr174 and the 3D model of ( Ta ) 5B2 predicts Tyr174 to play the same role as Thr160 plays in CDK2.

Evidence for communality in the primary determinants of CYP74 catalysis and of structural similarities between CYP74 and classical mammalian P450 enzymes.

In silico structural analysis of CYP74C3, a membrane-associated P450 enzyme from the plant Medicago truncatula (barrel medic) with hydroperoxide lyase (HPL) specificity, showed that it had strong similarities to the structural folds of the classical microsomal P450 enzyme from rabbits (CYP2C5). It was not only the secondary structure predictions that supported the analysis but site directed mutagenesis of the substrate interacting residues was also consistent with it. This led us to develop a substrate-binding model of CYP74C3 which predicted three amino acid residues, N285, F287, and G288 located in the putative I-helix and distal haem pocket of CYP74C3 to be in close proximity to the preferred substrate 13-HPOTE. These residues were judged to be in equivalent positions to those identified in SRS-4 of CYP2C5. Significance of the residues and their relevance to the model were further assessed by site directed mutagenesis of the three residues followed by EPR spectroscopic and detailed kinetic investigations of the mutated proteins in the presence and absence of detergent. Although point mutation of the residues had no effect on the haem content of the mutated proteins, significant effects on the spin state equilibrium of the haem iron were noted. Kinetic effects of the mutations, which were investigated using three different substrates, were dramatic in nature. In the presence of detergent with the preferred substrate (13-HPOTE), the catalytic center activities and substrate binding affinities of the mutant proteins were reduced by a factor of 8-32 and 4-12, respectively, compared with wild-type--a two orders of magnitude reduction in catalytic efficiencies. We believe this is the first report where primary determinants of catalysis for any CYP74 enzyme, which are fully consistent with our model, have been identified. Our working model predicts that N285 is close enough to suggest that a hydrogen bond with the peroxy group of the enzyme substrate 13-HPOTE is warranted, whereas significance of F287 may arise from a strong hydrophobic interaction between the alkyl group(s) of the substrate and the phenyl ring of F287. We believe that G288 is crucial because of its size. Any other residue with a relatively bulky side chain will hinder the access of substrate to the active site. The effects of the mutations suggests that subtle protein conformational changes in the putative substrate-binding pocket regulate the formation of a fully active monomer-micelle complex with low-spin haem iron and that structural communication exists between the substrate- and micelle-binding sites of CYP74C3. Conservation in CYP74 sequence alignments suggests that N285, F287, and G288 in CYP74C3 and the equivalent residues at positions in other CYP74 enzymes are likely to be critical to catalysis. To support this we show that G324 in CYP74D4 (Arabidopsis AOS), equivalent to G288 in CYP74C3, is a primary determinant of positional specificity. We suggest that the overall structure of CYP74 enzymes is likely to be very similar to those described for classical P450 monooxygenase enzymes.

Electron donation between copper containing nitrite reductases and cupredoxins: the nature of protein-protein interaction in complex formation.

In denitrifying organisms with copper containing dissimilatory nitrite reductases, electron donation from a reduced cupredoxin is an essential step in the reduction of nitrite to nitric oxide. Copper nitrite reductases are categorised into two subgroups based on their colour, green and blue, which are found in organisms where the cupredoxins are pseudoazurins and azurins, respectively. In view of this and some in vitro electron donation experiments, it has been suggested that copper nitrite reductases have specific electron donors and that electron transfer takes place in a specific complex of the two proteins. We report results from the first comprehensive electron donation experiments using three copper nitrite reductases, one green and two blue, and five cupredoxins, one pseudoazurin and four azurins. Our data show that pseudoazurin can readily donate electrons to both blue and green copper nitrite reductases. In contrast, all of the azurins react very sluggishly as electron donors to the green nitrite reductase. These results are discussed in terms of surface compatibility of the component proteins, complex formation, overall charges, charge distribution, hydrophobic patches and redox potentials. A docking model for the complexes is proposed.

Analysis of the nitric oxide-sensing non-heme iron center in the NorR regulatory protein.

The NorR regulatory protein senses nitric oxide (NO) to activate genes required for NO detoxification under anaerobic and microaerobic conditions in Escherichia coli. NorR belongs to the sigma(54)-dependent family of transcriptional activators and contains an N-terminal regulatory GAF (cGMP phosphodiesterase, adenylate cyclase, FhlA) domain that controls the ATPase activity of the central AAA+ domain to regulate productive interactions with sigma(54). Binding of NO to a non-heme iron center in the GAF domain results in the formation of a mononitrosyl-iron complex and releases intramolecular repression of the AAA+ domain to enable activation of transcription. In this study, we have further characterized NorR spectroscopically and substituted conserved residues in the GAF domain. This analysis, in combination with structural modeling of the GAF domain, has identified five candidate ligands to the non-heme iron and suggests a model in which the metal ion is coordinated in a pseudo-octahedral environment by three aspartate residues, an arginine, and a cysteine.

An SNF2 protein associated with nuclear RNA silencing and the spread of a silencing signal between cells in Arabidopsis.

The silencing phenotype in Arabidopsis thaliana lines with an inverted repeat transgene under the control of a phloem-specific promoter was manifested in regions around veins due to a mobile signal of silencing. Genetic analysis implicates RNA-DEPENDENT RNA POLYMERASE2 (RDR2) and an RNA polymerase IVa subunit gene (NRPD1a) in the signaling mechanism. We also identified an SNF2 domain-containing protein (CLASSY1) that acts together with RDR2 and NRPD1a in the spread of transgene silencing and in the production of endogenous 24-nucleotide short interfering RNAs (siRNAs). Cytochemical analysis indicates that CLASSY1 may act in the nucleus with NRPD1a and RDR2 in the upstream part of RNA silencing pathways that generate a double-stranded RNA substrate for Dicer-like (DCL) nucleases. DCL3 and ARGONAUTE4 act in a downstream part of the pathway, leading to endogenous 24-nucleotide siRNA production, but are not required for intercellular signaling. From genetic analysis, we conclude that another downstream part of the pathway associated with intercellular signaling requires DCL4 and at least one other protein required for 21-nucleotide trans-acting siRNAs. We interpret the effect of polymerase IVa and trans-acting siRNA pathway mutations in terms of a modular property of RNA silencing pathways.

Dithionite reduction kinetics of the dissimilatory copper-containing nitrite reductase of Alcalegenes xylosoxidans. The SO(2)(.-) radical binds to the substrate binding type 2 copper site before the type 2 copper is reduced.

We report here the first detailed study of the dithionite reduction kinetics of a copper-containing dissimilatory nitrite reductase (NiR). The reduction of the blue type 1 copper (T1Cu) center of NiR preparations that contained both type 1 and type 2 copper atoms, followed biphasic kinetics. In contrast, NiR that was deficient in type 2 copper (T2DNiR), followed monophasic kinetics with a second-order rate constant (T2D)k = 3.06 x 10(6) m(-1) s(-1). In all cases the SO(2)(.-) radical rather than S(2)O(4)(2-) was the effective reductant. The observed kinetics were compatible with a reaction mechanism in which the T1Cu of the fully loaded protein is reduced both directly by dithionite and indirectly by the type 2 Cu (T2Cu) site via intramolecular electron transfer. Reduction kinetics of the T2Cu were consistent with SO(2)(.-) binding first to the T2Cu center and then transferring electrons (112 s(-1)) to reduce it. As SO(2)(.-) is a homologue of NO(2)(-), the NiR substrate, it is not unlikely that it binds to the catalytic T2Cu site. Effects on the catalytic activity of the enzyme using dithionite as a reducing agent are discussed. Reduction of the semireduced T1Cu(I)T2Cu(II) state followed either second-order kinetics with k(2) = 3.33 x 10(7) m(-1) s(-1) or first-order kinetics with 52.6 s(-1) < (T1red)k(1) < 112 s(-1). Values of formation constants of the T1Cu(II)T2Cu(II)-SO(2)(.-) and T1Cu(I)T2Cu(II)-SO(2)(.-) adducts showed that the redox state of T1Cu affected binding of SO(2)(.-) at the catalytic T2Cu center. Analysis of the kinetics required the development of a mathematical protocol that could be applied to a system with two intercommunicating sites but only one of which can be monitored. This novel protocol, reported for the first time, is of general application.

Alcaligenes xylosoxidans dissimilatory nitrite reductase: alanine substitution of the surface-exposed histidine 139l ligand of the type 1 copper center prevents electron transfer to the catalytic center.

Nitrite reductase of Alcaligenes xylosoxidans contains three blue type 1 copper centers with a function in electron transfer and three catalytic type 2 copper centers. The mutation H139A, in which the solvent-exposed histidine ligand of the type 1 copper ion was changed to alanine, resulted in the formation of a colorless protein containing 4.4 Cu atoms per trimer. The enzyme was inactive with reduced azurin as the electron donor, and in contrast to the wild-type enzyme, no EPR features assignable to type 1 copper centers were observed. Instead, the EPR spectrum of the H139A enzyme, with parameters of g(1) = 2.347 and A(1) = 10 mT, was typical of type 2 copper centers. On the addition of nitrite, the EPR features developed spectral features with increased rhombicity, with g(1) = 2.29 and A(1) = 11 mT, arising from the type 2 catalytic site. As assessed by visible spectroscopy, ferricyanide (E degree = +430 mV) was unable to oxidize the H139A enzyme, and this required a 30-fold excess of K(2)IrCl(6) (E degree = +867 mV). Oxidation resulted in the EPR spectrum developing additional axial features with g(1) = 2.20 and A(1) = 9.5 mT, typical of type 1 copper centers. The oxidized enzyme after separation from the excess of K(2)IrCl(6) by gel filtration was a blue-green color with absorbance maxima at 618 and 420 nm. The instability of the protein prevented the precise determination of the midpoint potential, but these properties indicate that it is in the range 700-800 mV, an increase of at least approximately 470 mV compared with the native enzyme. This high potential, which is consistent with a trigonal planar geometry of the Cu ion, effectively prevents azurin-mediated electron transfer from the type 1 center to the catalytic type 2 Cu site. However, with dithionite as reductant, 20% of the activity of the wild-type enzyme was observed, indicating that the direct reduction of the catalytic site by dithionite can occur. When CuSO(4) was added to the crude extract before isolation of the enzyme, the Cu content of the purified H139A enzyme increased to 5.7 Cu atoms per trimer. The enzyme remained colorless, and the activity with dithionite as a donor was not significantly increased. The additional copper in such preparations was associated with an axial type 2 Cu EPR signal with g(1) = 2.226 and A(1) = 18 mT, and which were not changed by the addition of nitrite, consistent with the activity data.