Poly-beta-hydroxybutyrate metabolism is affected by changes in respiratoryenzymatic activities due to cold stress in two psychrotrophic strains of Rhizobium

Cold stress resulted in a decrease in the poly-beta -hydroxybutyrate (PHB) content of non-cold-acclimated Rhizobium DDSS69 cultures. Analysis of the s pecific activity of beta -ketothiolase and beta -hydroxybutyrate dehydrogen ase revealed that decrease in PHB levels was a result of the inhibition of synthesis of PHB rather than an increase in its breakdown. Rhizobium ATR1, a cold-acclimated strain, revealed the presence of a stable PHB metabolism that did not show any significant differences either in PHB levels or in th e activity of enzymes of the PHB metabolism under cold stress, suggesting t hat PHB is not involved in cold tolerance. Analysis of specific activities of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase o f the pentose phosphate pathway showed the upward regulation of alternate p athways of carbohydrate metabolism under cold stress to rapidly generate en ergy to overcome the stress. There is diversity in the switching mechanisms of carbon metabolism among cold-acclimated and non-cold-acclimated Rhizobi um isolates. Upward regulation of malate dehydrogenase in both isolates sug gests that it is a critical input for cold tolerance.