Identification of an Acidic Amino Acid Permease Involved in d-Aspartate Uptake in the Yeast Cryptococcus humicola

Analysis of Lactobacillus rhamnosus GG in Mulberry Galacto-Oligosaccharide Medium by Comparative Transcriptomics and Metabolomics

Lactobacillus rhamnosus GG (LGG) has strong acid resistance and can survive passing through the stomach to colonize the intestines, where it promotes the growth of beneficial bacteria. Prebiotics such as mulberry galacto-oligosaccharide (MGO), mulberry polysaccharide solution (MPS), and galactooligosaccharides (GOS) promote LGG proliferation, and MGO has the greatest effect. After culturing LGG with prebiotics, changes in gene expression were studied at the transcriptomic and metabolomic levels. The results showed that, in the stable 24-h growth period of cultivation, ~63 and 132% more differential genes were found after MPS and MGO were added to the MRS medium, respectively, than after GOS was added, and the numbers of up-regulated genes were about 18 and 66% higher with MPS and MGO, respectively, than GOS. Analysis using the KEGG database revealed that, when LGG was cultured with MGO, 120 genes that were up-regulated as the growth rate increased were mainly enriched in pathways such as membrane transport, amino acid metabolism, and carbohydrate metabolism. The genes gatB and gatC were up-regulated for galactose metabolism, and bglA was up-regulated in the glycolysis/gluconeogenesis pathway. The qRT-RCR results, which were in agreement with the RNA-seq, indicated the genes involved in the proliferation effect of LGG were up-regulated. UDP-glucose may be a key metabolite for MGO to promote LGG proliferation.

Regulation of d-Aspartate Oxidase Gene Expression by Pyruvate Metabolism in the Yeast Cryptococcus humicola

d-Aspartate oxidase (DDO) is a peroxisomal flavoenzyme that catalyzes the oxidative deamination of acidic d-amino acids. In the yeast Cryptococcus humicola strain UJ1, the enzyme ChDDO is essential for d-Asp utilization and is expressed only in the presence of d-Asp. Pyruvate carboxylase (Pyc) catalyzes the conversion of pyruvate to oxaloacetate and is involved in the import and activation of certain peroxisomal flavoenzymes in yeasts. In this study, we analyzed the role of Pyc in the expression of ChDDO gene in C. humicola strain UJ1. PYC gene disruption (∆Chpyc1) in strain UJ1 resulted in growth retardation on glucose and NH₄Cl medium. The growth was restored by supplying oxaloacetate from l-Asp or α-ketoglutarate by a transaminase. On the other hand, the supply of oxaloacetate from d-Asp by ChDDO was not able to prevent growth retardation because of a significant decrease in ChDDO gene expression at the transcriptional level. The addition of pyruvate significantly decreased ChDDO gene transcription in the ∆Chpyc1 strain but increased the same in the wild-type strain, even though the intracellular pyruvate content was similar in both strains. These results suggest that ChDDO gene expression might be regulated by pyruvate metabolism, as well as by the presence of d-Asp.