SUMMARY: Molybdenum uptake was examined in K12 using the radionuclide Mo. The molybdenum uptake system was characterized in an unusual strain, which appeared to be normal in uptake of the MoO ion but altered in subsequent molybdenum processing. As a consequence, molybdenum could be chased from cells in the strain, while it was irreversibly assimilated in the wild-type strain. Molybdenum uptake showed a biphasic kinetic curve, with a very rapid binding followed by a slow uptake phase. The uptake appeared to involve an active transport system. Molybdenum, probably in the form of molybdate, accumulated by a factor of about 30 in the cells. An energy source was necessary and uptake was inhibited by arsenate, but not by CCCP (carbonyl cyanide -chlorophenylhydrazone). The uptake system saturated with a K of 2.5-2.7 × 10 M. Uptake seemed to depend on a periplasmic binding protein, since cold shock treatment and arsenate abolished uptake. A molybdate binding protein activity was detected in the periplasmic fluid with a K of 9 nM. Sulphate inhibited uptake and the uptake activity was pH dependent, with an apparent p of 6.7. These results imply that molybdate transport belongs to the family of energy-dependent periplasmic binding protein systems. An explanation for the peculiar behaviour of the strain used in this work is proposed.


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