초록
<P>The functional sweetener, <SMALL>d</SMALL>-tagatose, is commonly transformed from galactose by <SMALL>l</SMALL>-arabinose isomerase. To make use of a much cheaper starting material, lactose, hydrolization, and isomerization are required to take place collaboratively. Therefore, a single-step method involving β-<SMALL>d</SMALL>-galactosidase was explored for <SMALL>d</SMALL>-tagatose production. The two vital genes, β-<SMALL>d</SMALL>-galactosidase gene (<I>lacZ</I>) and <SMALL>l</SMALL>-arabinose isomerase mutant gene (<I>araA</I>′) were extracted separately from <I>Escherichia coli</I> strains and incorporated into <I>E. coli</I> simultaneously. This gave us <I>E. coli</I>-ZY, a recombinant producing strain capable of coexpressing the two key enzymes. The resulted cells exhibited maximum <SMALL>d</SMALL>-tagatose producing activity at 34 °C and pH 6.5 and in the presence of borate, 10 mM Fe<SUP>2+</SUP>, and 1 mM Mn<SUP>2+</SUP>. Further monitoring showed that the recombinant cells could hydrolyze more than 95% lactose and convert 43% <SMALL>d</SMALL>-galactose into <SMALL>d</SMALL>-tagatose. This research has verified the feasibility of single-step <SMALL>d</SMALL>-tagatose fermentation, thereby laying down the foundation for industrial usage of lactose.</P><P><B>Graphic Abstract</B><BR><IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jafcau/2014/jafcau.2014.62.issue-11/jf4042485/production/images/medium/jf-2013-042485_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jf4042485'>ACS Electronic Supporting Info</A></P>