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 Lmalate 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. © 2013, American Society for Microbiology.