Results and Discussion

MRSLMASAALILALAMLFLSFPSEISANQYSYSSPPPPVHSPPPPKDPYHYSSPPPPP

VHTYPHPHPVY . ■ ■ ' : VHTYPHPHPVYHSPPPPTPHKKPYKYPSPPPPPVHTYPHPHPVYHSPPPP HKKPYKYSSPPPPPVHTYPHPHPVYHSPPPP VHTYPHPHPVYHSPPPPP TPHKKPYKYPSPPPP1AHTYPPHVPTPVYHSPPPPAYSPPPPAYYYKSPPPPYHH*

Figure 1. Consensus of translated MGP sequences deduced from 30 cDNA clones.

Figure 1 shows the consensus of amino acid sequences from N-terminal methionine to stop codon (*) deduced from 30 cDNA clones. The conserved N-terminal, "central" and C-terminal regions are shown in bold. The central conserved region can be divided into two motifs, each of which is reiterated in other parts of the sequence. Motif I, Thr-Pro-His-Lys-Lys-Pro-Tyr-Lys-Tyr-Pro-Ser-(Pro)5, is also present within the C-terminal domain. Motif II, Val-His-Thr-Tyr-Pro-His-Pro-His-Pro-Val-Tyr-His-Ser-(Pro)4, is repeated twice in the central domain and is also repeated a variable number of times elsewhere. Variant residues are underlined and blocks of SP4 or SP5 are shaded. Note that Tyr residues generally occur in pairs, except at the C-terminus. This might be a significant feature in relation to peroxide-driven cross-linking (see below).

MGP as an extensin corresponds most closely to the deduced amino-acid sequence of an extensin from V. faba, VfNDS-E, and from Medicago truncatula (Gamas et al 1996), both of which are apparently up-regulated in nodules relative to root tissues. We propose to term this sub-class of extensins "nodule extensins". Because nodule extensin is a key component of the extracellular matrix, we hypothesize that its physical and biochemical properties may have an important influence on the process of tissue and cell colonization by Rhizobium bacteria (Peters et al 2000).

The solubility of MGP (i.e. its extractability from tissues) is apparently reduced by hydrogen peroxide (Wisniewski et al. 1999). Protein cross-linking is also suggested by the fact that these antigens often have different apparent molecular masses, following isolation from different tissues of pea and lupin (de Lorenzo et al. 1998). These preliminary observations suggest that protein cross-linking may be an important factor controlling the fluidity of the matrix that surrounds rhizobia in the infection thread lumen. Propagation of the infection thread may depend on the dynamic equilibrium between secretion of nodule extensin into the lumen and insolubilization of this matrix by locally generated hydrogen peroxide (Wisniewski et al. 1999). During incompatible interactions, overproduction of peroxide might be expected to result in abortion of infection threads (Vasse et al. 1993).

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