Role of riboflavin biosynthesis gene duplication and transporter in Aeromonas salmonicida virulence in marine teleost fish

Active flavins derived from riboflavin (vitamin B₂) are essential for life. Bacteria biosynthesize riboflavin or scavenge it through uptake systems, and both mechanisms may be present. Because of riboflavin's critical importance, the redundancy of riboflavin biosynthetic pathway (RBP) genes might be present. Aeromonas salmonicida, the aetiological agent of furunculosis, is a pathogen of freshwater and marine fish, and its riboflavin pathways have not been studied. This study characterized the A. salmonicida riboflavin provision pathways. Homology search and transcriptional orchestration analysis showed that A. salmonicida has a main riboflavin biosynthetic operon that includes ribD, ribE1, ribBA, and ribH genes. Outside the main operon, putative duplicated genes ribA, ribB and ribE, and a ribN riboflavin importer encoding gene, were found. Monocistronic mRNA ribA, ribB and ribE2 encode for their corresponding functional riboflavin biosynthetic enzyme. While the product of ribBA conserved the RibB function, it lacked the RibA function. Likewise, ribN encodes a functional riboflavin importer. Transcriptomics analysis indicated that external riboflavin affected the expression of a relatively small number of genes, including a few involved in iron metabolism. ribB was downregulated in response to external riboflavin, suggesting negative feedback. Deletion of ribA, ribB and ribE1 showed that these genes are required for A. salmonicida riboflavin biosynthesis and virulence in Atlantic lumpfish (Cyclopterus lumpus). A. salmonicida riboflavin auxotrophic attenuated mutants conferred low protection to lumpfish against virulent A. salmonicida. Overall, A. salmonicida has multiple riboflavin endowment forms, and duplicated riboflavin provision genes are critical for A. salmonicida infection.

A second riboflavin import system is present in flavinogenic Streptomyces davaonensis and supports roseoflavin biosynthesis

The antibiotic roseoflavin is produced by Streptomyces davaonensis in the stationary phase of growth. To support biosynthesis of the secondary metabolite roseoflavin, S. davaonensis underwent several genetic adaptations with regard to metabolism of the roseoflavin precursor and primary metabolite riboflavin. In addition to 17 riboflavin biosynthesis genes at different chromosomal locations, S. davaonensis contains the riboflavin transporter gene ribM being part of the riboflavin biosynthetic operon ribE1MAB5H. Deletion of this operon generated riboflavin auxotrophic S. davaonensis strains. The finding that S. davaonensis ΔribE1MAB5H was able to grow in a culture medium containing low levels of riboflavin indicated that in addition to RibM, a second riboflavin transporter is present in this bacterium. The S. davaonensis genes ribXY (former rosXY) represented candidate genes for such a second riboflavin transport system and the results of our experiments now show that RibXY from S. davaonensis is a highly efficient riboflavin importer but not a roseoflavin importer.

Disorders of riboflavin metabolism

Riboflavin (vitamin B2), a water-soluble vitamin, is an essential nutrient in higher organisms as it is not endogenously synthesised, with requirements being met principally by dietary intake. Tissue-specific transporter proteins direct riboflavin to the intracellular machinery responsible for the biosynthesis of the flavocoenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These flavocoenzymes play a vital role in ensuring the functionality of a multitude of flavoproteins involved in bioenergetics, redox homeostasis, DNA repair, chromatin remodelling, protein folding, apoptosis, and other physiologically relevant processes. Hence, it is not surprising that the impairment of flavin homeostasis in humans may lead to multisystem dysfunction including neuromuscular disorders, anaemia, abnormal fetal development, and cardiovascular disease. In this review, we provide an overview of riboflavin absorption, transport, and metabolism. We then focus on the clinical and biochemical features associated with biallelic FLAD1 mutations leading to FAD synthase deficiency, the only known primary defect in flavocoenzyme synthesis, in addition to providing an overview of clinical disorders associated with nutritional deficiency of riboflavin and primary defects of riboflavin transport. Finally, we give a brief overview of disorders of the cellular flavoproteome. Because riboflavin therapy may be beneficial in a number of primary or secondary disorders of the cellular flavoproteome, early recognition and prompt management of these disorders is imperative.

Structure/functional aspects of the human riboflavin transporter-3 (SLC52A3): role of the predicted glycosylation and substrate-interacting sites

The human riboflavin (RF) transporter-3 (hRFVT-3; product of the SLC52A3 gene) plays an essential role in the intestinal RF absorption process and is expressed exclusively at the apical membrane domain of polarized enterocytes. Previous studies have characterized different physiological/biological aspects of this transporter, but nothing is known about the glycosylation status of the hRFVT-3 protein and role of this modification in its physiology/biology. Additionally, little is known about the residues in the hRFVT-3 protein that interact with the ligand, RF. We addressed these issues using appropriate biochemical/molecular approaches, a protein-docking model, and established intestinal/renal epithelial cells. Our results showed that the hRFVT-3 protein is glycosylated and that glycosylation is important for its function. Mutating the predicted N-glycosylation sites at Asn⁹⁴ and Asn¹⁶⁸ led to a significant decrease in RF uptake; it also led to a marked intracellular (in the endoplasmic reticulum, ER) retention of the mutated proteins as shown by live-cell confocal imaging studies. The protein-docking model used in this study has identified a number of putative substrate-interacting sites: Ser¹⁶, Ile²⁰, Trp²⁴, Phe¹⁴², Thr³¹⁴, and Asn³¹⁵ Mutating these potential interacting sites was indeed found to lead to a significant inhibition in RF uptake and to intracellular (ER) retention of the mutated proteins (except for the Phe¹⁴² mutant). These results demonstrate that the hRFVT-3 protein is glycosylated and this glycosylation is important for its function and cell surface expression. This study also identified a number of residues in the hRFVT-3 polypeptide that are important for its function/cell surface expression.