Les phytases hydrolysent les phytates représentant la forme majeure de stockage du P dans les céréales. Ces phytates sont aussi des facteurs anti-nutritionnels qui chélatent les cations réduisant leur absorption. Dans le premier volet de cette thèse, une nouvelle souche bactérienne produisant une phytase extracellulaire a été isolée et identifiée comme Bacillus amyloliquefaciens US573. L’enzyme «PHY US573» a été purifiée et caractérisée en comparaison avec deux phytases commerciales Ronozyme PL et Natuphos. PHY US573 se distingue par sa forte thermostabilité en présence de calcium. En outre, PHY US573 se caractérise aussi par une tolérance remarquable aux sels comme le NaCl et LiCl. L’ensemble de ces propriétés montre que PHY US573 pourrait être une candidate intéressante pour des applications en alimentation animale ou en agriculture pour améliorer la biodisponibilité du P-phytique pour les plantes. Dans le deuxième volet, la souche Streptomyces sp. US42 produisant une activité phytase extracellulaire a été sélectionnée. L’enzyme «PHY US42» a été purifiée et caractérisée. PHY US42 est calcium dépendante également une grande stabilité en présence de sels biliaires et des protéases digestives. La modélisation moléculaire de PHY US42 indique qu'elle appartient au groupe des β-propeller phytases qui sont généralement calcium-dépendantes. Vu ses propriétés biochimiques intéressantes, PHY US42 constitue une bonne candidate comme additif dans les aliments pour animaux monogastriques en combinaison avec une histidine acide phytase. Enfin dans un troisième volet, nous nous sommes intéressés à l’étude de la régulation de l'expression du gène phytase de S. coelicolor M145 (sco7697) chez S. coelicolor M145, S. lividans TK24 ainsi que chez ses deux mutants ppk et phoP. Ainsi, en plus des boites pho localisées en amont de la région promotrice -35 siège de la régulation positive PhoP-dépendante, nous avons révélé pour la première fois que la RD localisée en aval de la région promotrice -10 est le siège d’une forte régulation négative par un répresseur inconnu. Ce dernier empêcherait l’activation PhoP-dépendante de l’expression du gène phytase. / Phytases hydrolyse phytate representing the major storage form of P in cereal. phytates are also anti-nutritional factors that chelate cations such as Ca²⁺, Mg²⁺, Fe²⁺, Z²⁺ reducing their absorption. The low bioavailability of phytic phosphorus in monogastric animals require their food supplementation with Pi to meet the needs of the animal in P. This creates an extra cost and increases the environmental pollution by the manure excretion highly charged phosphate. In the first part of this thesis, from soil samples taken near hot hydrothermal waters of the region Elhamma in southern Tunisian, a new bacterial strain producing extracellular phytase was isolated and identified as Bacillus amyloliquefaciens US573. The enzyme referred "PHY US573" was purified and characterized in comparison with two commercial acid histidine phytases Ronozyme PL and Natuphos. PHY US573 is calcium dependent and has an optimum activity at pH 7.5 (5 for Ronozyme and 5.5 for Natuphos) and 70°C (55°C for Ronozyme and Natuphos). PHY US573 is distinguished by its high thermostability, in fact, it keeps 93% of its activity after incubation for 10 min at 75°C in the presence of calcium while Ronozyme and Natuphos keep only 45% and 53% of their activity, respectively. This enzyme is specific for phytic acid and also has a very good stability at pH 3 to 9 and a perfect stability in presence of bile salts. In addition, PHY US573 is also characterized by a remarkable salt tolerance because it retains 80 to 95% of its activity in the presence of 20 g/l of NaCl and LiCl, respectively. All these properties shows that PHY US573 could be an interesting candidate for applications in feed industry alone or in combination with an histidine acid phytase. In a second part of this thesis, from the Streptomyces collection of LMB-CBS, a strain producing extracellular phytase activity was selected and identified as Streptomyces sp. US42. The enzyme "PHY US42" was purified and characterized. PHY US42 has a calcium-dependent activity (such as Bacillus phytases), optimally active at pH 7 and 65°C. PHY US42 is perfectly stable at pHs ranging from 5 to 10 and its thermal stability is greatly increased in the presence of calcium. Indeed, PHY US42 maintains 80% of its activity after 10 min of incubation at 75 °C in the presence of calcium. PHY US42 has also a high stability in the presence of bile salts and digestive proteases. Molecular modeling of PHY US42 indicates that it belongs to the β-propeller phytase group which are usually calcium-dependent. Given its interesting biochemical properties, PHY US42 which would operate mainly in the intestine, is a good candidate for use as an additive in agastriques fish food or in combination with an histidine acid phytase in feed industry. Finally in a third part, we are interested in studying the regulation of the expression of the phytase gene of S. coelicolor M145 (sco7697) in S.coelicolor M145, S.lividans TK24 and among its two mutants ppk and phoP. To do this, we merged the wild promoter regions (phyWT) or mutated (phym1, phym2, phym1+2) of sco7697 gene with the GUS reporter gene encoding ß-glucuronidase activity. Thus as expected, we demonstrated that the deletion of the PHO box located upstream of the -35 reduces the level of induction of sco7697 in conditions of Pi limitation. Moreover, we have revealed for the first time that the alteration of RD located downstream of -10 correlates with a dramatic increase of GUS expression when PhoP is present. Our results demonstrate that this RD is the seat of a strong negative regulation by an unknown repressor. This would prevent the PhoP-dependent activation of expression of the phytase gene.