Organisms can employ temperature-specific alleles, or isozymes, as an adaptation to low temperatures by possessing two alleles of the same enzyme that have different temperature optima. While animals commonly use isozymes as an adaptation to temperature changes, few examples have been documented in bacteria (Ishii et al. 1987; He et al. 2001). Bacterial isozymes were first documented for isocitrate dehydrogenase of the psychrophile Colwellia maris (Ochiai et al. 1979, 1984; Ishii et al. 1987). While it is difficult to assess the extent to which isozymes occur throughout the genome of P. arcticus 273-4, putative isozymes have been identified in tran-scriptome experiments where the expression of one isozyme is increased at high temperatures and the expression of the second isozyme is increased at low temperatures (Bergholz et al., personal communication). For example, there are two genes for RNA helicase (1082 and 943). The transcript for 943 was upregulated during growth at warm temperatures, while 1082 was upregulated at low temperatures. Similar patterns of expression were documented for two dihydrolipoamide dehy-drogenases, two D-alanyl-D-alanyl carboxypeptidases, and two 16S rRNA pseudou-ridine synthases. Certainly, these enzymes are important to cell function at any temperature; hence, the presence of isozymes would ensure that these functions are maintained regardless of growth temperature. Additional data from analysis of the proteome and the phenotype of deletion mutants suggest that isozymes (for the substrate-binding subunits of ferric-citrate and dicarboxylic acid transporters) may be involved in ensuring that key nutrients are transported across the membrane at different temperatures (Bakermans et al. 2007). The use of isozymes may be particularly useful to microorganisms that live in permafrost given that their initial habitat, prior to burial, is within the active layer of permafrost where temperatures fluctuate on a seasonal — and sometimes daily — basis around the freezing point of water.
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