L. Kleihues et al., The H-2 sensor of Ralstonia eutropha is a member of the subclass of regulatory [NiFe] hydrogenases, J BACT, 182(10), 2000, pp. 2716-2724
Two energy-generating hydrogenases enable the aerobic hydrogen bacterium Ra
lstonia eutropha (formerly Alcaligenes eutrophus) to use molecular hydrogen
as the sole energy source. The complex synthesis of the nickel-iron-contai
ning enzymes has to be efficiently regulated in response to H-2, which is a
vailable in low amounts in aerobic environments. H-2 sensing in R. eutropha
is achieved by a hydrogenase-like protein which controls the hydrogenase g
ene expression in concert with a two-component regulatory system. In this s
tudy we show that the H-2 sensor of R. eutropha is a cytoplasmic protein. A
lthough capable of H-2 oxidation with redox dyes as electron accepters, the
protein did not support lithoautotrophic growth in the absence of the ener
gy-generating hydrogenases. A specifically designed overexpression system f
or R. eutropha provided the basis for identifying the H-2 sensor as a nicke
l-containing regulatory protein. The data support previous results which sh
owed that the sensor has an active site similar to that of prototypic [NiFe
] hydrogenases (A. J. Pierik, M. Schmelz, O. Lent, B. Friedrich, and S. P.
J. Albracht, FEES Lett. 438:231-235, 1998). It is demonstrated that in addi
tion to the enzymatic activity the regulatory function of the R, sensor is
nickel dependent. The results suggest that H-2 sensing requires an active:[
NiFe] hydrogenase, leaving the question open whether only H-2 binding or su
bsequent H-2 oxidation and electron transfer processes are necessary for si
gnaling. The regulatory role of the H-2-sensing hydrogenase of R. eutropha,
which has also been investigated in other hydrogen-oxidizing bacteria, is
intimately correlated with a set of typical structural features. Thus, the
family of H-2 sensors represents a novel subclass of [NiFe] hydrogenases de
noted as the "regulatory hydrogenases."