Multilocus Sequence Analysis Reveals Genetic Diversity in Xanthomonads Associated With Poinsettia Production.

Xanthomonas axonopodis pv. poinsettiicola is traditionally identified as the primary causal agent of bacterial leaf spot on poinsettia (family Euphorbiaceae). Sixty-seven strains of xanthomonads isolated from lesions associated with several species within the family Euphorbiaceae were collected over a 64-year period. The pathogenicity of these strains was compared on several potential hosts and they were analyzed by multilocus sequence analysis (MLSA) using six housekeeping genes. The 67 Xanthomonas strains associated with poinsettia production were separated into three distinct clades based on MLSA. The first clade identified contained the X. axonopodis pv. poinsettiicola reference strain (LMG849PT). A second clade was more closely related to X. hortorum pv. pelargonii (LMG7314PT) and the third clade contained the X. codiaei type strain (LMG8678T). This analysis indicated that there may also be other closely related pathovars or species of Xanthomonas that can infect poinsettia. Strains from the three clades could not be distinguished by symptoms or virulence on poinsettia plants. Strains capable of infecting geranium were found in all three clades, although the extent of leaf spot formation and number of systemic infections were significantly less than those produced by X. hortorum pv. pelargonii strains, typically the main causal agent of bacterial leaf spot on geranium. Clade III also contained strains isolated from zebra plant (Aphelandra squarrosa, family Acanthaceae), which is a newly recognized host for X. codiaei and X. axonopodis pv. poinsettiicola. Xanthomonas leaf spot is a serious threat to poinsettia production that can be caused by several Xanthomonas spp. that can infect different ornamental plant hosts. It is imperative that growers maintain a strict sanitation program because reservoirs of inoculum can occur on a number of ornamental hosts.

Modeling of deoxy- and dideoxyaldohexopyranosyl ring puckering with MM3(92).

Extensive variations of the ring structures of three deoxyaldohexopyranoses, L-fucose, D-quinovose, and L-rhamnose, and four dideoxyaldohexopyranoses, D-digitoxose, abequose, paratose, and tyvelose, were studied by energy minimization with the molecular mechanics algorithm MM3(92). Chair conformers, 4C(1) in D-quinovose and the equivalent 1C(4) in L-fucose and L-rhamnose, overwhelmingly dominate in the three deoxyhexoses; in the D-dideoxyhexoses, 4C(1) is again dominant, but with increased amounts of 1C(4) forms in the alpha anomers of the three 3,6-dideoxyhexoses, abequose, paratose, and tyvelose and in both alpha and beta anomers of the 2,6-dideoxyhexose D-digitoxose. In general, modeled proton-proton coupling constants agreed well with experimental values. Computed anomeric ratios strongly favor the beta configuration except for D-digitoxose, which is almost equally divided between alpha and beta configurations, and L-rhamnose, where the beta configuration is somewhat favored. MM3(92) appears to overstate the prevalence of the equatorial beta anomer in all three deoxyhexoses, as earlier found with fully oxygenated aldohexopyranoses.

Automated docking of alpha-(1-->4)- and alpha-(1-->6)-linked glucosyl trisaccharides and maltopentaose into the soybean beta-amylase active site.

The Lamarckian genetic algorithm of AutoDock 3.0 was used to dock alpha-maltotriose, methyl alpha-panoside, methyl alpha-isopanoside, methyl alpha-isomaltotrioside, methyl alpha-(6(1)-alpha-glucopyranosyl)-maltoside, and alpha-maltopentaose into the closed and, except for alpha-maltopentaose, into the open conformation of the soybean beta-amylase active site. In the closed conformation, the hinged flap at the mouth of the active site closes over the substrate. The nonreducing end of alpha-maltotriose docks preferentially to subsites -2 or +1, the latter yielding nonproductive binding. Some ligands dock into less optimal conformations with the nonreducing end at subsite -1. The reducing-end glucosyl residue of nonproductively-bound alpha-maltotriose is close to residue Gln194, which likely contributes to binding to subsite +3. In the open conformation, the substrate hydrogen-bonds with several residues of the open flap. When the flap closes, the substrate productively docks if the nonreducing end is near subsites -2 or -1. Trisaccharides with alpha-(1-->6) bonds do not successfully dock except for methyl alpha-isopanoside, whose first and second glucosyl rings dock exceptionally well into subsites -2 and -1. The alpha-(1-->6) bond between the second and third glucosyl units causes the latter to be improperly positioned into subsite +1; the fact that isopanose is not a substrate of beta-amylase indicates that binding to this subsite is critical for hydrolysis.