The Effect Of DNA Repair Protein RecO And Leucine Metabolism On The Tolerance Of Lactic Acid Bacteira To Multiple Stresses

Lactic acid bacteria (LAB) have to face with multiple stresses in the process of growthand industrial application. Acid stress, which is one of the most important stress LAB facedwith, seriously restricts the viability of cells and restrains the growth and metabolism of LAB.The addition of leucine is proved to improve the tolerance of Lactococcus lactis ssp. cremorisNZ9000to acid stress. When cultivated with leucine under acid stress (pH5.0), the biomassof L. lactis NZ9000(Leu+) is1.24-fold higher, compared to L. lactis NZ9000(Leu-)(withoutleucine addition). After challenged at pH4.0for5h, the survival rate of L. lactis NZ9000(Leu+) is28.5-fold higher than the corresponding value of L. lactis NZ9000(Leu-). Furthermeasurements indicate that the addition of leucine could improve the concentration ofintracellular NH4+, help to maintain the intracellular pH (pHin) at a relatively higher level andprotect the activity of lactate dehydrogenase (LDH). Therefore, the addition of leucine couldenhance the acid tolerance of L. lactis.Multiple stresses could probably cause damage to DNA and other macromolecules.RecO, belonging to the family of DNA repair proteins, plays an important part in homologousrecombination and replication repair. Consequently, in order to explore the role of RecO inovercoming multiple stresses, a mutant of recO deletion is constructed in L. lactis NZ9000byusing a Cre-lox-based system. In the presence of RecO, the original strain L. lactis NZ9000shows better performances in growth under multiple stresses. Compared with the mutantstrain L. lactis NZ9000ΔrecO, the survival rates of original strain under acid, osmotic andchill stresses are13.49-,2.78-and60.89-fold higher. In our deeper research on fermentationcapability under osmotic stress, the maximum lactate acid production and maximum lactateproductivity of L. lactis NZ9000are1.28-and1.5-fold higher, respectively, and the lactatedehydrogenase (LDH) activity of L. lactis NZ9000is1.63-fold higher after8-hourfermentation, which suggests that the fermentation efficiencies of the original strain maintainat a higher level. Further study on the transcription of key genes related to stress responseshows that genes of DNA repair proteins and some heat shock proteins (such as recA，recR，groEL and grpE) are over-reglulated. The anaplerotic flux of recO in L. lactis NZ9000ΔrecOshows that growth performances and viabilities of L. lactis NZ-RecO are dramaticly higherthan the corresponding values of L. lactis NZ-Vector. These results indicate that RecO haspositive function for the survival of L. lactis NZ9000under multiple stresses, and contributesto protect its key enzymes. Our research confirms the role of RecO in enhancing tolerances tomultiple stresses of L. lactis NZ9000.These discoveries would be of benefit to not only lactic acid bacteria, but other industrialmicroorganisms that the anti-stress components confirmed by this study could be used toimprove their resistances to various stresses.