Background: In plants and photosynthetic bacteria, the tyrosine degradation pathway is crucial because homogentisate, a tyrosine degradation product, is a precursor for the biosynthesis of photosynthetic pigments, such as quinones or tocophenols. Homogentisate biosynthesis includes a decarboxylation step, a dioxygenation and a rearrangement of the pyruvate sidechain, This complex reaction is carried out by a single enzyme, the 4-hydroxyphenylpyruvate dioxygenase (HPPD), a non-heme iron dependent enzyme that is active as a homotetramer in bacteria and as a homodimer in plants. Moreover, in humans, a HPPD deficiency is found to be related to tyrosinemia, a rare hereditary disorder of tyrosine catabolism,Results: We report here the crystal structure of Pseudomonas fluorescens HPPD refined to 2.4 Angstrom resolution (R-free 27.6%; R factor 21.9%). The general topology of the protein comprises two barrel-shaped domains and is similar to the structures of Pseudomonas 2,3-dihydroxybiphenyl dioxygenase (DHBD) and Pseudomonas putida catechol 2,3-dioxygenase (MPC), Each structural domain contains two repeated beta alpha beta beta beta alpha modules. There is one non-heme iron atom per monomer liganded to the sidechains of His161, His240, Glu322 and one acetate molecule,Conclusions: The analysis of the HPPD structure and its superposition with the structures of DHBD and MPC highlight some important differences in the active sites of these enzymes, These comparisons also suggest that the pyruvate part of the HPPD substrate (4-hydroxyphenylpyruvate) and the O-2 molecule would occupy the three free coordination sites of the catalytic iron atom. This substrate-enzyme model will aid the design of new inhibitors of the homogentisate biosynthesis reaction.

A model is proposed to describe the genetic value of 4x-2x hybrids resulting from crosses between tetraploid genitors and diploid genitors that produce 2n gametes. The model takes into account the genetic consequences of the First Division Restitution (FDR) and Second Division Restitution (SDR) meiosis, particularly on the homozygosity level that 2n gametes contribute to 4x-2x hybrids. As genes can be identical by descent, numerous parameters are needed in the classical approach to describe the inbreeding effects on the mean and variance of 4x-2x hybrids. Using the concept of test value, the model allows a large decrease in the number of required parameters. The model gives the components of genetic variance and usual covariances between relatives using these synthetic parameters. The model is then used to study the efficiency of a recurrent breeding scheme to improve diploid genitors for their combining ability with tetraploid genitors. It appears that, in presence of dominance, ignoring the meiosis pattern will lead to an overestimation of additive variance and then of genetic advance. Some genetic considerations on the differences between FDR and SDR mechanisms lead us to suggest an experimental comparison of their respective advantages and disadvantages for the type of considered recurrent selection. An experimental crossing design is proposed to obtain estimates of the genetic parameters needed for this comparison.

Le modele theorique de genetique quantitative de decomposition des effets genetiques chez les tetraploides est applique aux descendances tetraploides de croisements u. S. P. Et b. S. P. (uni-bilateral sexuel polyploidisation). Un modele intra-population pour les croisements chez tuberosum et un modele inter-population pour les croisements inter-specifiques sont developpes. Les moyennes, les covariances entre apparentes, les estimations des parametres genetiques, les variances intra-croisement, les effets du desequilibre panmictique, les heritabilites des differents types de croisements sont calcules et permettent leurs comparaisons deux a deux. Les consequences au niveau d'un programme de selection sont discutees. Un essai sur deux ans permet de comparer les croisements 4x-2x 2n for, 4x-2x 2n syn, et 4x-4x sur les moyennes et les variances intra-croisement pour les caracteres lies au rendement. Un essai sur deux ans pour le choix de geniteurs tetraploides sur descendances par un testeur 2x2n syn, pour les caracteres de rendement est etudie. Les resultats de ces essais sont discutes pour juger la valadite du modele theorique, les avantages et les problemes poses par l'utilisation de la voie diploide pour l'amelioration de la pomme de terre