The rhizobial genes needed to establish a symbiosis have been identified, and some are listed (Table 14.8). Genes referred to as common nodulation genes (nod A, B, C, and D) are found in all rhizobia and are essential for initiation of nodulation. In addition, many other genes that define the legume host range of specific rhizo-bia have been identified and are referred to as nod, or nol, or noe genes. These genes are involved in the synthesis and transport of an unusual class of compounds referred to as "nod factors" that induce nodule formation. Nod factors are composed of an oligosaccharide backbone of (31-4-linked N-acetylglucosamine residues with a fatty acid acyl group attached to the N atom of the nonreducing acetyl glucosamine residue (Fig. 14.5). These compounds are referred to as lipochitin oligosaccharides or LCOs. LCOs usually contain three to six N-acetyl-glucosamine residues. The common nod genes A, B, and C encode enzymes that play key roles in synthesis of the LCO backbone structure and include chitin oligosaccharide synthase (nodC), chitin oligosaccharide deacetylase (nodB), and acyltransferase (nodA). A large number of structural variants exist among the LCOs (Table 14.9). Host specificity is based upon the structural variations among the nod factors. A number of specific substituents ("decorations") are found on positions R1 through R9, and different rhizobial genes (referred to as host-range genes) are involved in the attachment of these moieties. For example, the placement of a SO4 group at position R5 of the LCO from Sinorhizobium meliloti is essential for nodulation of alfalfa (M. sativa), whereas O-methyl fucosylation of R5 is essential for nodulation of soybean (Glycine max) by Bradyrhizobium japon-icum. Nod factors are produced by rhizobia in response to inducers secreted by germinating seedlings and by plant roots. The most potent inducers are flavones
TABLE 14.8 Examples of Nodulation (nod ) Genes and Their Proposed Functions"
Regulatory genes nodDj nodV nodW nolA nolR syrM
Nod factor core synthesis nodA nodB nodC nodM
Nod factor core modifications nodE nodF nodH nodL nodS nodL nodP nodZ nolL nolO noeC noel
Transcriptional activator Two-component regulator Two-component regulator Transcriptional regulator Repressor, DNA binding protein Transcriptional regulator
Acetyltransferase Deacetylase Chitin synthase D-Glucosamine synthase
Ketoacyl synthase Acyl carrier protein Sulfotransferase Acetyl transferase Methyl transferase ATP-sulfurylase ATP-sulfurylase Fucosyl transferase O-acetyltransferase Carbamoyl transferase Arabinosylation 2-O-methylation
Integral membrane protein Outer membrane transport Pore-forming protein aThe list of genes is not inclusive; some genes coding for the same function in different organisms are coded differentially.
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