| Summary: | In <i>Lactococcus lactis</i> and some other lactic acid bacteria, respiratory metabolism has been reported upon supplementation with only heme, leading to enhanced biomass formation, reduced acidification, resistance to oxygen, and improved long-term storage. Genes encoding a complete respiratory chain with all components were found in genomes of <i>L. lactis</i> and <i>Leuconostoc mesenteroides</i>, but menaquinone biosynthesis was found to be incomplete in Lactobacillaceae (except <i>L. mesenteroides</i>). <i>Lactiplantibacillus plantarum</i> has only two genes (<i>menA</i>, <i>menG</i>) encoding enzymes in the biosynthetic pathway (out of eight), and <i>Lentilactobacillus buchneri</i> has only four (<i>menA</i>, <i>menB</i>, <i>menE</i>, and <i>menG</i>). We constructed knock-out strains of <i>L. lactis</i> defective in <i>menA</i>, <i>menB</i>, <i>menE</i>, and <i>menG</i> (encoding the last steps in the pathway) and complemented these by expression of the extant genes from <i>Lactipl. plantarum</i> and <i>Lent. buchneri</i> to verify their functionality. Three of the <i>Lactipl. plantarum</i> biosynthesis genes, <i>lpmenA1</i>, <i>lpmenG1</i>, and <i>lpmenG2</i>, as well as <i>lbmenB</i> and <i>lbmenG</i> from <i>Lent. buchneri</i>, reconstituted menaquinone production and respiratory growth in the deficient <i>L. lactis</i> strains when supplemented with heme. We then reconstituted the incomplete menaquinone biosynthesis pathway in <i>Lactipl. plantarum</i> by expressing six genes from <i>L. lactis</i> homologous to the missing genes in a synthetic operon with two inducible promoters. Higher biomass formation was observed in <i>Lactipl. plantarum</i> carrying this operon, with an OD<sub>600</sub> increase from 3.0 to 5.0 upon induction.
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