MOLYBDENUM UPTAKE IN ESCHERICHIA-COLI K12

Molybdenum uptake was examined in Escherichia coli K12 using the radio nuclide Mo-99. The molybdenum uptake system was characterized in an un usual chlD strain, which appeared to be normal in uptake of the MoO42- ion but altered in subsequent molybdenum processing. As a consequence , molybdenum could be chased from cells in the chlD strain, while it w as irreversibly assimilated im the wild-type strain. Molybdenum uptake showed a biphasic kinetic curve, with a very rapid binding followed b y a slow uptake phase. The uptake appeared to involve an active transp ort system. Molybdenum, probably in the form of molybdate, accumulated by factor of about 30 in the cells. An energy source was necessary an d uptake was inhibited by arsenate, but not by CCCP (carbonyl cyanide m-chlorophenylhydrazone). The uptake system saturated with a K(m) of 2 .5-2.7 x 10(-8) M. Uptake seemed to depend on a periplasmic binding pr otein, since cold shock treatment and arsenate abolished uptake. A mol ybdate binding protein activity was detected in the periplasmic fluid with a K(D) of 9 nM. Sulphate inhibited uptake and the uptake activity was pH dependent, with an apparent pK of 6.7. These results imply tha t molybdate transport belongs to the family of energy-dependent peripl asmic binding protein systems. An explanation for the peculiar behavio ur of the chlD strain used in this work is proposed.