UC San Diego

Biological membranes are highly dynamic complex structures exquisitely tuned to alterations in the state of the physical environment. Increased hydrostatic pressure and reduced temperature elicit similar physical influences on the phase and fluidity properties of biological membranes. As hydrostatic pressure is increased, or temperature is decreased, membranes undergo a reversible change from a fluid disordered state to a non-fluid ordered state resulting in membrane supraoptimal viscosity or phase transition. These deleterious consequences result to a large extent from the close packing of phospholipid fatty acyl chains within the membrane bilayer. In the absence of corrective membrane restructuring events to alleviate this perturbation, a host of membrane localized essential processes are prone to inactivation. Consequently, preservation of a suitable membrane dynamic state is of paramount importance in allowing an organism to adapt accordingly to prevailing environmental conditions.

In this dissertation, the relative importance of unsaturated fatty acids (UF As) to growth at high pressure and low temperature has been evaluated employing the piezophilic deep-sea bacterium Photobacterium profundum strain SS9. SS9 modulates the levels of numerous fatty acid species in response to pressure and temperature change resulting in enhanced levels of UF As at high pressure and low temperature. Analysis of SS9 mutant strains exhibiting altered fatty acid profiles have revealed particular classes of UFA species important for growth at high pressure and low temperature. Furthermore, molecular genetic analyses of genes responsible for UFA synthesis have identified multiple genes required for pressure-responsive UFA production. The fatty acid biosynthetic condensing enzymes FabF (KAS II) and FabB (KAS I) are required for high pressure regulation of 18: 1 synthesis and growth at elevated pressure. In addition, the genes responsible for omega-3 polyunsaturated fatty acid (PUF A) synthesis in SS9 have been characterized. PUF A levels undergo both pressure- and temperature-dependent modulations yet they are not required for growth under these conditions. Lastly, the involvement of the global transcription factor CRP is evidenced to influence UFA synthesis in SS9. Overall, these studies present a comprehensive analysis of membrane-based adaptations important for growth and survival under elevated pressure and low temperature conditions.