The TonB-dependent uptake of pyrroloquinoline-quinone (PQQ) and secretion of gluconate by Escherichia coli K-12.

Glucose is taken up by Escherichia coli through the phosphotransferase system (PTS) as the preferred carbon source. PTS mutants grow with glucose as a carbon source only in the presence of pyrroloquinoline quinone (PQQ), which is needed as a redox cofactor for the glucose dehydrogenase Gcd. The membrane-anchored Gcd enzyme oxidises glucose to gluconolactone in the periplasm. For this reaction to occur, external supply of PQQ is required as E. coli is unable to produce PQQ de novo. Growth experiments show that PqqU (previously YncD) is the TonB-ExbBD-dependent transporter for PQQ through the outer membrane. PQQ protected the cells from the PqqU-dependent phage IsaakIselin (Bas10) by competition for the receptor protein. As a high affinity uptake system, PqqU allows E. coli to activate Gcd even at surrounding PQQ concentrations of about 1 nmoL/L. At about 30-fold higher PQQ concentrations, the activation of Gcd gets PqqU independent. Due to its small size, Pqq may also pass the outer membrane through porins. The PQQ-dependent production of gluconate has been demonstrated in many plant growth-promoting bacteria that solubilise phosphate minerals in the soil by secreting this acid. Under phosphate limiting conditions also E. coli induces the glucose dehydrogenase and secretes gluconate, even in absence of PTS, that is, even when the bacterium is unable to grow on glucose without PQQ.