6533b871fe1ef96bd12d2671

RESEARCH PRODUCT

Malic Enzyme and Malolactic Enzyme Pathways Are Functionally Linked but Independently Regulated in Lactobacillus casei BL23

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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.