By Ivan Gyulev
A study published in October 2016 in Nature Chemical Biology by Prof Morten Sommer and colleagues reported the use of a sequence-independent screen for the identification of novel bacterial small molecule transporters. The assay is based on a synthetic selection system that relies on riboswitch biosensors. A riboswitch (small molecule-binding RNAs) is located in the 5’UTR of an antibiotic resistance gene and inhibits its translation by sequestering its ribosome binding sites. However, when the riboswitch’s ligand is present in sufficient concentration intracellularly, the translational repression is alleviated and the gene is expressed, thereby conferring resistance against its respective antibiotic. By using two antibiotics and two resistance genes, the researchers dramatically reduced the rate of false positive mutants. Using this assay, one can screen a library of metagenomic fragments for ligand importers. To screen for importers of a new ligand researchers only need to change the riboswitch. Genee et al. demonstrated the modularity of their design by implementing it in the discovery of thiamine and xanthine importers.
The outline of the synthetic selection system in the case of selection for thiamine importers (using the ThiM19 riboswitch) is shown below (taken from Figure 1a from the paper).
After validating the synthetic selection system the authors then screened metagenomic DNA libraries from soil and gut fecal samples for thiamine importers and discovered a novel class of thiamine importer – PnuT (screen strategy outlined in Figure 2a from the paper).
PnuT has homology to the nicotinamide riboside and nicotinamide mononucleotide transporter PnuC and had been previously predicted to be involved in thiamine uptake. PnuT’s function as a thiamine transporter was validated by selective growth and intracellular thiamine quantification by HPLC. Further bioinformatics analysis, revealed that PnuT is very common in the Bacteroidetes phylum. The authors then looked at phylogeny and the pattern of Pnu transporters’ co-localization with genes from thiamine salvage or biosynthesis pathway across genomes.
Finally, a previously published synthetic riboswitch (derived from aptamer identified by SELEX) was utilized to select for xanthine importers. The screen resulted in the isolation of two unique ORFs with more than 99% sequence identity at the amino acid level with known xanthine permeases from the NAT/NCS2 nucleobase-ascorbate transporter family. In both screens, fragments containing multi-drug resistance proteins were isolated
The authors highlight several limitations of the current screen technique –firstly, the discovery of transporters relying on multiple protein complexes (such as the thiamine importer from Bacillus/Clostridium ECF-ThiT or the E.coli ThiBPQ) would require larger metagenomic (or genomic) fragments (in the present study the range was between 1kb and 3kb but it is possible to use larger fragments). Secondly, these proteins are not necessarily encoded in the same chromosomal region. Thirdly, naturally-occurring riboswitches and allosteric transcription factors are the go-to choice for small-molecule biosensors but synthetic riboswitches are more difficult to develop synthetically. Reportedly, one way to go around this is to construct a metabolic pathway bridging an undetectable compound to a detectable one.
Altogether, the novel synthetic selection strategy is a powerful tool for the isolation and validation of novel importers from metagenomic libraries or putative transporters from genomic sequences. It is also impressive that in its first implementation the assay led to the experimental validation of a novel import system.
Source: Genee, H.J. et al., (2016).Functional mining of transporters using synthetic selections. Nat. Chem. Biol. 12, 1015-1022.