Disruption of the signal recognition particle pathway leading to impaired growth, sugar metabolism and acid resistance of Lactococcus lactis

Lactococcus lactis is a well-known workhorse for dairy products, whose important industrial traits are tightly associated with numerous cytoplasmic membrane proteins. However, roles of the signal recognition particle (SRP) pathway responsible for membrane protein targeting have not been studied in L. lactis. In this work, the putative genes ffh and ftsY encoding SRP pathway components were identified in the genome of L. lactis NZ9000. Experimental evidence showed that sequence mutation in either the ffh or ftsY was not lethal, but prolonged the lag phase of the resultant mutants Δffh and ΔftsY by 2 h and lowered their biomass to 85.7 % of the wild type under static conditions, as well as deprived the mutants of improved growth capacity under aerobic respiration conditions. Besides, the speeds of glucose consumption and lactate production were significantly decreased in the mutants. Then, the impact of the SPR components on acid resistance was detected, showing that the ffh and ftsY were transcriptionally upregulated by 3.02 ± 1.21 and 8.66 ± 1.01-fold in the wild type during acid challenge at pH 3.0, and cell survival of the Δffh and ΔftsY decreased by10- and 100-fold compared with the wild type. To explore the possible mechanism about the SRP pathway involved in the above physiological traits, proteomics analysis was performed and revealed that disruption of the Ffh or FtsY led to decrease in ribosomal proteins, but increase in DnaK, GroEL and heat shock protein GrpE, indicating that the SRP pathway was closely linked to protein synthesis and folding in L. lactis. Decrease in the fructose-bisphosphate aldolase, respiratory complexes NADH dehydrogenase, as well as glutamate decarboxylase was also detected in the Δffh and ΔftsY, which is consistent with the phenomena of impaired sugar metabolism and acid resistance. Our results demonstrated the dispensable SRP pathway could contribute to the maintenance of metabolism homeostasis and acid resistance of L. lactis.