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.