Étude des gènes dupliqués de la glycolyse chez lactococcus lactis il1403

Abstract

-Study of the duplicated glycolytic genes in Lactococcus lactis IL1403. The conversion of sugars into lactic acid is the main metabolic pathway providing energy to lactic acid bacteria. This conversion is also involved in production of different compounds participating to the organoleptic properties of fermented products. The L. lactis knowledge of the genome has given the access to sequences of genes encoding the enzymes involved in the two main metabolic pathways described for the fermentation of glucose in lactic acid bacteria: (1) the homofermentative pathway through glycolysis leading to two lactate molecules per glucose consumed; (2) the heterofermentative pathway through the Pentose Phosphate pathway giving one lactate, one acetate and one CO 2 per molecule of glucose. The research of the genes, corresponding to proteins involved in these metabolic pathways, revealed that some enzymes are encoded by 2 distinct genes. This fact could give to the cell the possibility to produce enzymes with different biochemical properties, or to control their expression according to specific conditions. Two copies of genes potentially encoding glyceraldehyde-3-phosphate dehydrogenase (gap) and enolase (eno) have been identified. Other microorganisms such as E. coli and B. subtilis also possess 2 gap genes sharing up to 60% homology, but having different functions. In L. lactis, gap1 and gap2 genes share around 80% identity at both the nucleotidic and protein level. The analysis of codon usage, the transcription and the effect of genes inactivation shows that gap1 is the only gene involved in glycolysis. The transcription of this essential gene is very high during all phases of growth. Low increase of the level of transcription could be evidenced during growth in glucose, a sugar inducing the Catabolite Repression. Moreover, the presence of potential fixation site for CcpA (Cre box) upstream of initiation transcription box -35 suggests that gap1 transcription is activated by this protein. In contrast, the gap2 gene is dispensable and expressed at a very low level in our experimental conditions. Finally, in opposition to GapB from B. subtilis, the product of L. lactis gap2 might not to be involved in the neoglucogenesis. enoA and enoB genes are coding for proteins sharing 55% identity with known enolase. In opposition to the gap genes, the eno genes does not share significant nucleotidic homologies together. However, enoA presents 87% identity with the enolase genes from sequenced Streptococcus species whereas enoB presents 95% identity with a plasmidic encoded gene isolated from Streptococcus thermophilus. These observations suggest that enoB was transferred from species to other. The analysis of codons bias strongly suggests that EnoA is the

Comparative genomics of the lactic acid bacteria

“…lactis ssp. lactis have enzymatic activity (38); however, their specific physiological functions remain unknown. Many other genes for proteins involved in sugar metabolism and transport were duplicated early in the evolution of the Lactobacillales, including phosphoenolpyruvate phosphotransferase systems, ␤-galactosidase, GpmB family sugar phosphatases, galactose mutarotase, and L-lactate dehydrogenases of two distinct classes.…”