Analysis and modeling of substrate uptake and product release by prokaryotic and eukaryotic cells

Metabolite trafficking enables membrane-impermeable-terpene secretion by yeast

Metabolites are often unable to permeate cell membranes and are thus accumulated inside cells. We investigate whether engineered microbes can exclusively secrete intracellular metabolites because sustainable metabolite secretion holds a great potential for mass-production of high-value chemicals in an efficient and continuous manner. In this study, we demonstrate a synthetic pathway for a metabolite trafficking system that enables lipophilic terpene secretion by yeast cells. When metabolite-binding proteins are tagged with signal peptides, metabolite trafficking is highly achievable; loaded metabolites can be precisely delivered to a desired location within or outside the cell. As a proof of concept, we systematically couple a terpene-binding protein with an export signal peptide and subsequently demonstrate efficient, yet selective terpene secretion by yeast (~225 mg/L for squalene and ~1.6 mg/L for β-carotene). Other carrier proteins can also be readily fused with desired signal peptides, thereby tailoring different metabolite trafficking pathways in different microbes. To the best of our knowledge, this is the most efficient cognate pathway for metabolite secretion by microorganisms.

The engineering of metabolite secretion from microorganisms can lead to many applications in synthetic biology. In this article, the authors engineer a metabolite trafficking system for the secretion of medicinal terpenes.

Functional analysis of the promoter of the mitochondrial phosphate carrier human gene: identification of activator and repressor elements and their transcription factors

The phosphate carrier (PiC) catalyses the import of phosphate into mitochondria where it is needed for ATP synthesis. We have analysed the 5'-flanking region of the human PiC gene and found that it has a single transcriptional initiation site and lacks a TATA box. Through deletion analysis of the -1213/-25 nt region, we identified an activation domain (-223/-25) and an inhibition domain (-1017/-814). The most effective promoter activity in transfected HeLa cells corresponded to the region containing putative binding sites for Sp1 (-163/-142; where Sp1 stands for stimulating protein-1) and CREB (-138/-116; where CREB stands for cAMP-response-element-binding protein). These DNA sequences were active in gel-shift assays in the presence of HeLa cell nuclear extracts or recombinant Sp1 and CREB respectively. Forskolin increased PiC promoter activity via the CREB site. Both footprinting and transfection of deletion constructs of the inhibition region (-1017/-814) showed that PiC silencer activity extends over 25 nt (-943/-919), which specifically binds two proteins present in HeLa cell nuclear extracts. These transcription factors were purified by DNA affinity, analysed by MS and identified as p54(nrb)/NonO (nuclear RNA binding protein) and PSF (protein-associated splicing factor). The PiC silencer region cloned in front of the ferritin promoter conferred a strong inhibition to the heterologous promoter. These findings may provide insight into control of PiC gene expression in different cell types and under different growth conditions. To our knowledge, this is the first study to analyse the regulation of the PiC gene expression in any cell.