title: Malic enzyme and malolactic enzyme pathways are functionally linked but independently regulated in Lactobacillus casei BL23.
abstract: Lactobacillus casei is the only lactic acid bacterium in which two pathways for l-malate degradation have been described: the malolactic enzyme (MLE) and the malic enzyme (ME) pathways. Whereas the ME pathway enables L. casei to grow on l-malate, MLE does not support growth. The mle gene cluster consists of three genes encoding MLE (mleS), the putative l-malate transporter MleT, and the putative regulator MleR. The mae gene cluster consists of four genes encoding ME (maeE), the putative transporter MaeP, and the two-component system MaeKR. Since both pathways compete for the same substrate, we sought to determine whether they are coordinately regulated and their role in l-malate utilization as a carbon source. Transcriptional analyses revealed that the mle and mae genes are independently regulated and showed that MleR acts as an activator and requires internalization of l-malate to induce the expression of mle genes. Notwithstanding, both l-malate transporters were required for maximal l-malate uptake, although only an mleT mutation caused a growth defect on l-malate, indicating its crucial role in l-malate metabolism. However, inactivation of MLE resulted in higher growth rates and higher final optical densities on l-malate. The limited growth on l-malate of the wild-type strain was correlated to a rapid degradation of the available l-malate to l-lactate, which cannot be further metabolized. Taken together, our results indicate that L. casei l-malate metabolism is not optimized for utilization of l-malate as a carbon source but for deacidification of the medium by conversion of l-malate into l-lactate via MLE.
authors.affiliation: Departamento de Biotecnología de Alimentos, Instituto de Agroquímica y Tecnología de Alimentos, Consejo Superior de Investigaciones Científicas, Paterna, Valencia, Spain.
authors.forename: José María
authors.initials: JM
authors.lastname: Landete
authors.forename: Sergi
authors.initials: S
authors.lastname: Ferrer
authors.forename: Vicente
authors.initials: V
authors.lastname: Monedero
authors.forename: Manuel
authors.initials: M
authors.lastname: Zúñiga
mesh_terms: D001692:Biological Transport; D002244:Carbon / Q000378:metabolism; D004734:Energy Metabolism; D020869:Gene Expression Profiling; D015964:Gene Expression Regulation, Bacterial; D055786:Gene Knockout Techniques; D019344:Lactic Acid / Q000378:metabolism; D007780:Lacticaseibacillus casei / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism*; D008291:Malate Dehydrogenase / Q000378:metabolism*; D008293:Malates / Q000378:metabolism*; D053858:Metabolic Networks and Pathways / Q000235:genetics*; D005810:Multigene Family
chemical_list: D008293:Malates; D019344:Lactic Acid; D002244:Carbon; C030298:malic acid; C037731:malolactic enzyme; D008291:Malate Dehydrogenase; C030574:malate dehydrogenase (decarboxylating)