Variability in gene content and expression of the thioredoxin system in Oenococcus oeni.

The thioredoxin system protects against oxidative stress through the reversible oxidation of the thioredoxin active center dithiol to a disulphide. The genome of Oenococcus oeni PSU-1 contains three thioredoxin genes (trxA1, trxA2, trxA3), one thioredoxin reductase (trxB) and one ferredoxin reductase (fdr) which, until recently, was annotated as a second thioredoxin reductase. For the first time, the entire thioredoxin system in several O. oeni strains isolated from wine has been analysed. Comparisons at the DNA and protein levels have been undertaken between sequences from O. oeni and other genera and species, and the genera Leuconostoc and Lactobacillus were found to present the highest similarities. The gene most frequently absent from a collection of 34 strains and the sequences annotated in the NCBI database was trxA1. Moreover, phylogenetic analysis suggested that this gene was horizontally transferred from Lactobacillus to O. oeni. Strain-dependent expression profiles were determined in rich and in wine-like media. General over-expression was detected after inoculation into wine-like medium, with trxA3 being the most highly expressed gene. The increased transcriptional levels of the thioredoxin genes are indicative of the crucial role of this system in the O. oeni response to wine harsh conditions.

Identification of variable genomic regions related to stress response in Oenococcus oeni.

The lactic acid bacterium Oenococcus oeni is the most important species involved in malolactic fermentation due to its capability to survive in presence of ethanol and in the acidic environment of wine. In order to identify novel genes involved in adaptation to wine, a new approach using genome-wide analysis based on stress-related genes was performed in strain O. oeni PSU-1, and 106 annotated stress genes were identified. The in silico analysis revealed the high similarity of all those genes through 57 O. oeni genomes; however, seven variable regions of genomic plasticity could be determined for their different presence observed among these strains. Regions 3 and 5 had the typical hallmarks of horizontal transfer, suggesting that the strategy of acquiring genes from other bacteria enhanced the fitness of O. oeni strains. Certain genes related to stress resistance were described in these regions, and similarities of putative acquired regions with other lactic acid bacteria species were found. Some genomic fragments present in all the strains were described and another new genomic island harbouring a threonine dehydrogenase was found. The association of selected sequences with adaptation to wine was assessed by screening 31 O. oeni strains using PCR of single genes, but no sequences were found to be exclusive to highly performing malolactic fermentation strains. This study provides new information about the genomic variability of O. oeni strains contributing to a further understanding of this species and the relationship of its genomic traits with the ability to adapt to stress conditions.