초록
<P><B>Background</B></P><P>Citric acid, a commodity product of industrial biotechnology, is produced by fermentation of the filamentous fungus <I>Aspergillus niger</I>. A requirement for high-yield citric acid production is keeping the concentration of Mn<SUP>2+</SUP> ions in the medium at or below 5 µg L<SUP>−1</SUP>. Understanding manganese metabolism in <I>A. niger</I> is therefore of critical importance to citric acid production. To this end, we investigated transport of Mn<SUP>2+</SUP> ions in <I>A. niger</I> NRRL2270.</P><P><B>Results</B></P><P>we identified an <I>A. niger</I> gene (<I>dmtA</I>; NRRL3_07789), predicted to encode a transmembrane protein, with high sequence identity to the yeast manganese transporters Smf1p and Smf2p. Deletion of <I>dmtA</I> in <I>A. niger</I> eliminated the intake of Mn<SUP>2+</SUP> at low (5 µg L<SUP>−1</SUP>) external Mn<SUP>2+</SUP> concentration, and reduced the intake of Mn<SUP>2+</SUP> at high (> 100 µg L<SUP>−1</SUP>) external Mn<SUP>2+</SUP> concentration. Compared to the parent strain, overexpression of <I>dmtA</I> increased Mn<SUP>2+</SUP> intake at both low and high external Mn<SUP>2+</SUP> concentrations. Cultivation of the parent strain under Mn<SUP>2+</SUP> ions limitation conditions (5 µg L<SUP>−1</SUP>) reduced germination and led to the formation of stubby, swollen hyphae that formed compact pellets. Deletion of <I>dmtA</I> caused defects in germination and hyphal morphology even in the presence of 100 µg L<SUP>−1</SUP> Mn<SUP>2+</SUP>, while overexpression of <I>dmtA</I> led to enhanced germination and normal hyphal morphology at limiting Mn<SUP>2+</SUP> concentration. Growth of both the parent and the deletion strains under citric acid producing conditions resulted in molar yields (Y<SUB>p/s</SUB>) of citric acid of > 0.8, although the deletion strain produced ~ 30% less biomass. This yield was reduced only by 20% in the presence of 100 µg L<SUP>−1</SUP> Mn<SUP>2+</SUP>, whereas production by the parent strain was reduced by 60%. The Y<SUB>p/s</SUB> of the overexpressing strain was 17% of that of the parent strain, irrespective of the concentrations of external Mn<SUP>2+</SUP>.</P><P><B>Conclusions</B></P><P>Our results demonstrate that <I>dmtA</I> is physiologically important in the transport of Mn<SUP>2+</SUP> ions in <I>A. niger</I>, and manipulation of its expression modulates citric acid overflow.</P>