Abstract
The phosphorylated form of thiamine (Vitamin B1), thiamine pyrophosphate (TPP) is essential for the metabolism of amino acids and carbohydrates in all organisms. Plants and microorganisms, such as yeast, synthesize thiamine de novo whereas animals do not. The thiamine signal transduction (THI) pathway in Saccharomyces cerevisiae is well characterized. The ~10 genes required for thiamine biosynthesis and uptake are transcriptionally upregulated during thiamine starvation by THI2, THI3, and PDC2. Candida glabrata, a human commensal and opportunistic pathogen, is closely related to S. cerevisiae but is missing half of the biosynthetic pathway, which limits its ability to make thiamine. We investigated the changes to the THI pathway in C. glabrata, confirming orthologous functions. We found that C. glabrata is unable to synthesize the pyrimidine subunit of thiamine as well as the thiamine precursor vitamin B6. In addition, THI2 (the gene encoding a transcription factor) is not present in C. glabrata, indicating a difference in the transcriptional regulation of the pathway. Although the pathway is upregulated by thiamine starvation in both species, C. glabrata appears to upregulate genes involved in thiamine uptake to a greater extent than S. cerevisiae. However, the altered regulation of the THI pathway does not alter the concentration of thiamine and its vitamers in the two species as measured by HPLC. Finally, we demonstrate potential consequences to having a partial decay of the THI biosynthetic and regulatory pathway. When the two species are co-cultured, the presence of thiamine allows C. glabrata to rapidly outcompete S. cerevisiae, while absence of thiamine allows S. cerevisiae to outcompete C. glabrata. This simplification of the THI pathway in C. glabrata suggests its environment provides thiamine and/or its precursors to cells, whereas S. cerevisiae is not as reliant on environmental sources of thiamine.
Highlights
Thiamine, or vitamin B1, is composed of two ring structures, a thiazole (4-methyl-5-β-hydroxyethylthiazole, HET) and a pyrimidine (2-methyl-4-amino-5-hydroxymethylpyrimidine, HMP) [1]
The loss of conserved synteny of the SNO/SNZ/THI5 gene families and the increased numbers of orthologs of these genes suggests that there was a selective advantage in the Saccharomyces clade to having increased thiamine and PLP biosynthesis and that C. glabrata does not benefit from increased thiamine production via HMP synthesis
Comparison of the THI pathways of S. cerevisiae and C. glabrata have allowed for a number of observations that suggest the two species thrive in different niches
Summary
Vitamin B1, is composed of two ring structures, a thiazole (4-methyl-5-β-hydroxyethylthiazole, HET) and a pyrimidine (2-methyl-4-amino-5-hydroxymethylpyrimidine, HMP) [1]. Thiamine is pyrophosphorylated, resulting in thiamine pyrophosphate (TPP), which is the active cofactor in amino acid and carbohydrate metabolism [2]. Identified as cocarboxylase, it is required for most in vivo decarboxylation reactions. Saccharomyces cerevisiae synthesizes TPP de novo through steps of condensation, hydrolysis, and pyrophosphorylation of HMP-PP and HET-P [4] (Fig 1A). HMP synthesis utilizes the proteins encoded by genes SNO2/3 and SNZ2/3, which are regulated by thiamine availability, to make the vitamin B6 ( known as pyridoxal-5’-phosphate or PLP) [4,5]. THI6 condenses HMP-PP and HET-P to thiamine phosphate. The final steps of TPP synthesis require the dephosphorylation and pyrophosphorylation of thiamine phosphate. The pyrophosphorylation is carried out by thiamine pyrophosphokinase and is encoded by THI80 [4,11]
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