-From genome to industrial application. The possibility to use modified microorganisms constructed by genetic engineering for food production in the next future will radically change our approach to optimize processes and improve the quality of the corresponding products. The research holding on this domain should allow the construction of strains more resistant to phage attack, producing new molecules (e.g. beneficial for health) or modified for their metabolism (growth, production of metabolites, resistance to stress, etc). The construction of such strains can be achieved by genetic engineering, either by modifying a pathway, or by expressing new genes. The engineering of known pathways allowed the development of new process and sorne of them are technically ready to be applied in the industry. For example, the expression of the anabolic acetolactate synthase, the inactivation of the LDH or of the acetolactate decarboxylase lead to the redirection of pyruvate to diacetyl instead of lactate. This might lead to a JO-fold increase in diacetyl production, and there are still sorne possibilities of improvement. Another project of metabolic engineering that is quite advanced, is the modification of proteolysis in Lactococcus lactis, including the degradation of casein, peptides and amino acids. Most genes involved in the assimilation of proteins have been characterized (cell-wall protease, peptides transporters, and peptidases). Isogenic strains with different level of peptidase activities are built in order to improve peptidolysis or change the pattern of its product. In addition to the modification of already existing pathways, the introduction of new pathways in a cell could brighten the metabolic possibilities of bacteria and lead to the construction of new strains, producing new aroma for example. Building a new pathway generally requires the expression of heterologous genes. However, the certitude that a gene is absent supposes that there is an exhaustive knowledge of the bacterial genome, as sorne genes may be cryptic or uninduced under the conditions tested in the laboratory. Although the important progresses in the research related to lactic acid bacteria in the last years, only few pathways have been weil characterized, covering, in terms of genetic information, a small percentage of the real metabolic possibilities. The size of the chromosome of most lactic acid bacteria is usually 1.8-3.4 Mb, about the half of the one of Escherichia coli, Bacillus subtilis and of sorne soil bacteria which have broad metabolic possibilities. On the other side, it is 4-5 fold the size of the smallest known genome, suggesting that the metabolic potentialities of lactic acid bacteria are underestimated. An investigation of data present in databases * Correspondance et tirés à part. shows that only 6% of the genome of L. lactis are available. These data co ver genes involved in amino acids and base biosynthesis (33%), degradation of peptides (17%), carbon catabolism (16%), stress responses (13%) and the remaining gen...
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