Ribulose bisphosphate carboxylase of the procaryotic symbiont of a hydrothermal vent tube worm: kinetics, activity and gene hybridization

Abstract The giant tube worm, Riftia pachyptila , which is abundant at deep-sea hydrothermal vents, contains an extremely high density of bacterial symbionts in a specialized 'trophosome' tissue. Although the symbiont has not been cultured, enzymatic studies by others indicate that the symbiont is capable of hydrogen-sulfide- or sulfur-based lithoautotrophy and fixes CO₂ via the Calvin-Benson cycle. Here we report additional findings for a specimen from the Guaymas Basin vent site (Gulf of California, 2000 m). Under assay conditions where activity was proportional to cell-free extract concentration, ribulose bisphosphate carboxylase/oxygenase (RuBisCO) activity was 6.3 nmol CO₂/mg protein per min (30°C). This is within the range observed for non-CO₂ limited cultures of sulfur bacteria. The activity vs. temperature profile suggests that the symbiont is a mesophile and not a thermophile. A substrate saturation curve shows an apparent K _m (with respect to ribulose 1,5-bisphosphate) of 65 μM which is considerably lower than the single previous report for a sulfur bacterial symbiont. Strong hybridization was detected between a gene probe derived from the RuBisCO large subunit gene of Anacystis nidulans and Riftia trophosome DNA. A Rhodospirillum rubrum -derived probe also showed hybridization with the same restriction fragments of symbiont DNA.