초록
<P><B>Background</B></P><P>Flavonoids are bio-active specialized plant metabolites which mainly occur as different glycosides. Due to the increasing market demand, various biotechnological approaches have been developed which use <I>Escherichia coli</I> as a microbial catalyst for the stereospecific glycosylation of flavonoids. Despite these efforts, most processes still display low production rates and titers, which render them unsuitable for large-scale applications.</P><P><B>Results</B></P><P>In this contribution, we expanded a previously developed in vivo glucosylation platform in <I>E. coli</I> W, into an efficient system for selective galactosylation and rhamnosylation. The rational of the novel metabolic engineering strategy constitutes of the introduction of an alternative sucrose metabolism in the form of a sucrose phosphorylase, which cleaves sucrose into fructose and glucose 1-phosphate as precursor for UDP-glucose. To preserve these intermediates for glycosylation purposes, metabolization reactions were knocked-out. Due to the pivotal role of UDP-glucose, overexpression of the interconverting enzymes <I>galE</I> and <I>MUM4</I> ensured the formation of both UDP-galactose and UDP-rhamnose, respectively. By additionally supplying exogenously fed quercetin and overexpressing a flavonol galactosyltransferase (F3GT) or a rhamnosyltransferase (RhaGT), 0.94 g/L hyperoside (quercetin 3-<I>O</I>-galactoside) and 1.12 g/L quercitrin (quercetin 3-<I>O</I>-rhamnoside) could be produced, respectively. In addition, both strains showed activity towards other promising dietary flavonols like kaempferol, fisetin, morin and myricetin.</P><P><B>Conclusions</B></P><P>Two <I>E. coli</I> W mutants were engineered that could effectively produce the bio-active flavonol glycosides hyperoside and quercitrin starting from the cheap substrates sucrose and quercetin. This novel fermentation-based glycosylation strategy will allow the economically viable production of various glycosides.</P><P><B>Electronic supplementary material</B></P><P>The online version of this article (doi:10.1186/s12934-015-0326-1) contains supplementary material, which is available to authorized users.</P>