NADP⁺ -dependent isocitrate dehydrogenase isozymes from a psychrotrophic bacterium, Psychrobacter sp. strain 13A

The genes encoding dimeric and monomeric isocitrate dehydrogenase (IDH) isozymes from a psychrotrophic bacterium, strain 13A (13AIDH-D and 13AIDH-M, respectively), were cloned and sequenced. The deduced amino acid sequences of these two IDHs showed high degrees of identity with those of bacteria of genus Psychrobacter. Analysis of the 16S ribosomal RNA gene of the strain 13A revealed that this bacterium is classified to genus Psychrobacter. The optimum temperatures for activities of 13AIDH-D and 13AIDH-M were 55°C and 45°C, respectively, indicating that they are mesophilic. On the contrary, 13AIDH-D maintained 90% of its maximum activity after incubation for 10 min at 50°C, while the 13AIDH-M activity was completely lost under the same condition. In addition, 13AIDH-D showed much higher specific activity than 13AIDH-M. From northern and western blot analyses, the 13AIDH-D gene was found to be not transcribed under the growth conditions tested in this study. However, the catalytic ability of the mesophilic 13AIDH-M was concluded to be enough to sustain the growth of strain 13A at low temperatures. Therefore, a novel pattern of the contribution of IDH isozymes in cold-living bacteria to their growth at low temperatures was confirmed in strain 13A.