Impairment of Lactococcus Iactis Growth, Sugar Metabolism, and Acid Resistance Due to Disruption of the Signal Recognition Particle Pathway

Abstract

The well-known dairy product workhorse Lactococcus lactis has significant industrial characteristics
that are closely linked to a variety of cytoplasmic membrane proteins. However, no research has been
done on the functions of the signal recognition particle (SRP) pathway, which targets membrane
proteins, in Lactobacillus lactis. This study found the putative genes ffh and ftsY that encode
components of the SRP pathway in the genome of Lactobacillus lactis NZ9000. Experiments revealed
that while neither the ffh nor the ftsY sequence mutation was fatal, it did lengthen the lag phase of the
resulting mutants Δffh and ΔftsY by two hours, reduce their biomass to 85.7% of the wild type in static
conditions, and prevent the mutants from growing more efficiently in aerobic respiration. In addition,
the mutants exhibited markedly reduced rates of lactate generation and glucose consumption. The
effect of the SPR components on acid resistance was then identified, revealing that during acid
challenge at pH 3.0, the ffh and ftsY were transcriptionally upregulated by 3.02 ± 1.21 and 8.66 ± 1.01
times in the wild type, respectively, and that the Δffh and ΔftsY cell survival dropped by 10 and 100
times in comparison to the wild type. Proteomics analysis was conducted to investigate the potential
mechanism of the SRP pathway involved in the aforementioned physiological traits. The results showed
that disruption of the Ffh or FtsY resulted in an increase in DnaK, GroEL, and heat shock protein GrpE,
but a decrease in ribosomal proteins. This suggests that the SRP pathway was closely associated with
protein synthesis and folding in Lactobacillus lactis. Reduced levels of respiratory complexes NADH
dehydrogenase, fructose-bisphosphate aldolase, and glutamate decarboxylase were also seen in the
Δffh and ΔftsY, which is in line with the phenomena of acid resistance and poor sugar metabolism. Our
findings showed that the dispensable SRP pathway may support L. lactis's acid resistance and
metabolism balance.