Heme synthesis in the rhizobium-legume symbiosis: A palette for bacterial and eukaryotic pigments

Rhizobia elicit nitrogen-fixing nodules on legume roots composed of highly differentiated plant and bacterial cells. Sustaining symbiosis is energy intensive, and an increase in plant and bacterial heme protein synthesis is an essential feature of nodule ontogeny. Symbiotic bacteroids express cytochrome heme proteins as part of an electron transport system that allows vigorous and efficient respiration in the low-O-2 milieu of nodules to support nitrogen fixation. Bacteroid respiration is also dependent on leghemoglobin, an abundant plant protein in nodules that facilitates O-2 diffusion to the endosymbiont. Interest in heme synthesis in the rhizobium-legume symbiosis has been stimulated by speculation that the heme prosthetic group of leghemoglobin is synthesized by the bacterial symbiont. As will be discussed, recent work suggests that this hypothesis is incorrect, but the idea has inspired experiments that have uncovered novel mechanisms for heme synthesis in nodules, These studies indicate that the seemingly esoteric processes involved in symbiotic development are variants of themes found in other prokaryotes and eukaryotes. Because they accent these themes, the symbiotic systems are excellent models for their study and have contributed to the refinement of paradigms that integrate these processes into the framework of biology.