Molecular Characterization and Regulation of Ammonia Assimilation in Chemoautotrophic Prokaryote-Eukaryote Symbioses
Final rept. 1 Mar 97-30 Nov 97
HARVARD UNIV CAMBRIDGE MA
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Metabolic integration and mechanisms of nutritional exchange are important but poorly understood aspects of the physiology of symbioses between chemoautotrophic bacteria and marine invertebrates. The ability to generate organic nitrogen compounds from inorganic sources is a vital physiological capability involving coordination of host and symbiont metabolism. Glutamine synthetase GS catalyzes the first step in the incorporation of inorganic nitrogen, as anmenia, into organic nitrogen compounds. The relative importance of host and symbiont GS in nitrogen assimilation was determined in several representative mollusk and tubeworm symbioses. In most cases, the primary activity present in nitrogen-assimilating tissues is that of symbiont GS, a GSI similar to that of free-living Gram-negative bacteria. In contrast, in a coastal bivalve symbiosis, host GS predominated with symbiont GS expressed at low levels or inactivated. Further studies of symbiont GS expression were conducted to determine whether symbiont physiology is sensitive to alterations in energy supply to the host. Expression of symbiont GS, as well as symbiont densities and expression of Rubisco, was unchanged in deep-sea vent tubeworms subjected to short-term energy sulfide starvation but dramatically altered by chronic exposure to low sulfide. These findings are suggestive of short-term host buffering of symbiont nutritional supply as well as mechanisms of reducing symbiont populations in response to long-term stresses.