Outer Membrane-Targeted Secretion of Respiration-Linked Fe(III) Terminal Reductases

Georgia Tech Research Corporation, Atlanta GA

Investigators

Abstract

Dissimilatory Fe(III) reduction is a relatively recent addition to the suite of anaerobic respiratory processes carried out by microorganisms. Compared to the wealth of knowledge existing on the molecular details of aerobic respiration, denitrification, sulfate reduction and methanogenesis, little is known about dissimilatory Fe(III) reduction. Ironically, recent microbiological and geological evidence indicates that dissimilatory Fe(III) reduction may have been one of the first respiratory processes to have evolved on early Earth. In the modern biosphere, dissimilatory Fe(III) reduction is central to a variety of globally significant processes. Recent genetic studies have indicated that Type II protein secretion is required for Fe(III) respiration activity by Shewanella putrefaciens. In Type II secretion mutants, a 91 kDa heme-containing, Fe(III) reductase is missing from the set of proteins loosely attached to the outside face of the S. putrefaciens outer membrane. S. putrefaciens may therefore overcome the physiological problem of respiring on insoluble Fe(III) substrates by using the Type II secretion system to target the 91 kDa heme-containing Fe(III) reductase to the outside face of the outer membrane where it acts as a respiration-linked, Fe(III) terminal reductase. A set of molecular cloning and biochemical analyses will be carried out to address this hypothesis. Anaerobic bacteria that respire (breathe) particulate iron (solid rust) may have been the first organisms to "breathe" on early Earth. Ironically, little is known about the details of this process. The respiratory system of aerobic bacteria (and humans) is located inside the cell because oxygen is able to diffuse through the cell wall and contact the respiratory system. Anaerobic bacteria that breathe on solid rust are therefore presented with a unique problem: the solid rust is unable to enter the cell and contact the respiratory system. The project will demonstrate that "rust breathing" bacteria overcome this problem by putting their respiratory system on the cell outer surface where it is able to contact the solid rust and generate energy.