hemoprotein ͉ nitric oxide ͉ signaling ͉ NMR ͉ phosphorylation

The xenognostic mechanisms of two multi-host pathogens, the causative agent of crown gall tumors Agrobacterium tumefaciens and the parasitic plant Striga asiatica, are compared. Both organisms are general plant pathogens and require similar information prior to host commitment. Two mechanistic strategies, chemical perception and metabolic complementation, are used to ensure successful host commitment. The critical reactions at host-parasite contact are proton and electron transfer events. Such strategies may be common among multi-host pathogens.

SummaryOver the last several years, intermediates in the reduction of dioxygen have been attributed diverse functional roles ranging from protection against pathogen attack to the regulation of cellular development. Evidence now suggests that parasitic angiosperms, which naturally commit to virulence through the growth of new organs, depend on reduced oxygen intermediates, or reactive oxygen species (ROS), for signal generation. Clearly, the role of ROS in both plant defense and other physiological responses complicates any models that employ these intermediates in host plant recognition. Here we exploit the transparent young Striga asiatica seedling to (i) localize the site of H 2 O 2 accumulation to the surface cells of the primary root meristem, (ii) demonstrate the accumulation of H 2 O 2 within cytoplasmic and apoplastic compartments, and (iii) document precise regulation of H 2 O 2 accumulation during development of the host attachment organ, the haustorium. These studies reveal a new active process for signal generation, host detection and commitment that is capable of ensuring the correct spatial and temporal positioning for attachment.

NafY participates in the final steps of nitrogenase maturation, having a dual role as iron-molybdenum cofactor (FeMoco) carrier and as chaperone to the FeMo-co-deficient apoNifDK (apo-dinitrogenase). NafY contains an N-terminal domain of unknown function (n-NafY) and a C-terminal domain (core-NafY) necessary for FeMo-co binding. We show here that n-NafY and core-NafY have very weak interactions in intact NafY. The NMR structure of n-NafY reveals that it belongs to the sterile ␣-motif (SAM) family of domains, which are frequently involved in protein-protein interactions. The presence of a SAM domain in NafY was unexpected and could not be inferred from its amino acid sequence. Although SAM domains are very commonly found in eukaryotic proteins, they have rarely been identified in prokaryotes. The n-NafY SAM domain binds apo-NifDK. As opposed to full-length NafY, nNafY impaired FeMo-co insertion when present in molar excess relative to FeMo-co and apo-NifDK. The implications of these observations are discussed to offer a plausible mechanism of FeMo-co insertion. NafY domain structure, molecular tumbling, and interdomain motion, as well as NafY interaction with apo-NifDK are consistent with the function of NafY in FeMo-co delivery to apo-NifDK. Complex metalloclusters are generated from simple [Fe-S] clusters by incorporating additional metals and organic/inorganic ligands. In the case of the FeMo-co of nitrogenase, simple [Fe-S] clusters are the substrates of the NifB protein, which uses them to generate NifB-co, a complex [Fe-S] cluster with a proposed [6Fe-