Xanthomonas oryzae pv. oryzae is a bacterial pathogen that causes leaf blight, a serious disease of rice. Most members of the genus Xanthomonas produce yellow, membrane bound, brominated aryl polyene pigments called xanthomonadins whose functional role is unclear. We find that pigment-deficient mutants of X. oryzae pv. oryzae exhibit hypersensitivity to photobiological damage. A clone containing the xanthomonadin biosynthetic gene cluster alleviates the hypersensitivity of the pigment-deficient mutant. Extracts containing xanthomonadin provide protection against photodynamic lipid peroxidation in liposomes. These results lead us to suggest a role for the pigment, namely protection against photodamage.

␤-Carbolines are tricyclic nitrogen heterocycles formed in plants and animals as Maillard reaction products between amino acids and reducing sugars or aldehydes. They are being detected increasingly in human tissues, and their physiological roles need to be understood. Two ␤-carboline carboxylates have been reported to accumulate in the human eye lens. We report here on the identification of another ␤-carboline, namely 1-methyl-1-vinyl -2,3,4-trihydro-␤-carboline-3-carboxylic acid, in the lenses of some cataract patients from India. Analysis of these three lenticular ␤-carbolines using photodynamic and antioxidant assays shows all of them to be inert as sensitizers and effective as antioxidants; they quench singlet oxygen, superoxide and hydroxyl radicals and inhibit the oxidative formation of higher molecular weight aggregates of the test protein, eye lens ␥-crystallin. Such antioxidative ability of ␤-carbolines is of particular relevance to the lens, which faces continual photic and oxidative stress. The ␤-carboline diacid IV is also seen to display an unexpected ability of inhibiting the thermal coagulation of ␥-crystallin and the dithiothreitol-induced precipitation of insulin. These results offer experimental support to earlier suggestions that one of the roles that the ␤-carbolines have is to offer protection against oxidative stress to the human tissues where they accumulate.

The ␤-and ␥-crystallins are closely related lens proteins that are members of the ␤␥-crystallin superfamily, which also include many non-lens members. Although ␤-crystallin is known to be a calcium-binding protein, this property has not been reported in ␥-crystallin. We have studied the calcium binding properties of ␥-crystallin, and we show that it binds 4 mol eq of calcium with a dissociation constant of 90 M. It also binds the calcium-mimic spectral probes, terbium and Stains-all. Calcium binding does not significantly influence protein secondary and tertiary structures. We present evidence that the Greek key crystallin fold is the site for calcium ion binding in ␥-crystallin. Peptides corresponding to Greek key motif of ␥-crystallin (42 residues) and their mutants were synthesized and studied for calcium binding. These peptides adopt ␤-sheet conformation and form aggregates producing ␤-sandwich. Our results with peptides show that, in Greek key motif, the amino acid adjacent to the conserved aromatic corner in the "a" strand and three amino acids of the "d" strand participate in calcium binding. We suggest that the ␤␥ superfamily represents a novel class of calcium-binding proteins with the Greek key ␤␥-crystallin fold as potential calcium-binding sites. These results are of significance in understanding the mechanism of calcium homeostasis in the lens.Calcium homeostasis plays an important role in lens transparency, opacification, and cataractogenesis. Cataracts can occur both under hypocalcemic and hypercalcemic conditions, so the actual amount of available calcium in the lens is an important parameter for the health of the lens (1, 2). The normal mammalian lens has around 0.2 mM total calcium, of which the amount of free Ca 2ϩ is only of the order of a few micromolars. Thus there must exist calcium regulation systems in the lens, and it is of interest to identify what they are and how they change in health and in disease. Vrensen et al. (3) have done an ultrastructural analysis of calcium distribution in the rat lens and have found calcium precipitates in the intermediate cortex fiber membranes, cytoplasm, and the nuclear envelope and very low levels of calcium in gap junctions, epithelial cells, and superficial fibers (3-5). The question of what the calcium-binding and -storing agents are in the lens is open; phospholipids and crystallins have been thought of as candidates. The major components of the lens are cytosolic proteins, crystallins, which account for about 40% of the wet weight of the lens. It is worth investigating whether any of the crystallins could act as a calcium sponge or storage depot in the tissue, particularly since the ultrastructural analysis shows calcium distribution in the cytoplasm. We have earlier shown that the ␤-and avian core protein ␦-crystallins show significant calcium-binding ability (6, 7). Thus, the possibility of crystallins acting as lenticular calcium-sequestering and -storing systems exists. However, the calcium binding properties of ␥-crystallin have not yet been...

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A general design strategy for the synthesis of cystine-based peptide nanotubes is described. The design essentially involves closing of the polymethylene chains with cystine diOMe. The cystine-based nanotubes are constructed by the self-assembly of a simple cyclobisamide building block, a key structural feature of which is the presence of two amide groups at almost opposite poles of the ring. A large variety of cyclobisamides with the general structure cyclo(−CO−(CH2) n −CO−Cyst−) have been prepared by a single-step procedure involving the condensation of 1,ω-alkane dicarbonyl dichloride [(CH2) n (COCl)2, n = 2, 3, ..., 10, 20] with cystine diOMe providing macrocyclic bisamides with ring size varying from 14 to 30 members. Single-crystal X-ray studies with four members (n = 4, 6, 8, and 10, respectively) have shown that the polymethylene-bridged cystine-based cyclobisamides possess the intrinsic property of self-assembling into highly ordered parallel arrays of solid-state nanotubes. The hydrogen-bonded cystine tubes are hollow and open ended and extend to infinity. The interior of the tubes is totally hydrophobic. As a result, the polymethylene-bridged peptide tubes (a) are able to enhance the solubility of highly lipophilic compounds in water, as demonstrated here, with pyrene and perylene polycyclic arenes, (b) are able to bind to fluorescent probe dyes such as Nile Red, and (c) can even induce an ordered secondary structure in linear peptides as shown here with the 26-residue bee-venom peptide melittin, in the 30-membered cystine tubule. Crystallographic parameters are (C14H22N2O6S2, P21212) a = 16.489(1) Å, b = 23.049(1) Å, c = 4.864(1) Å; (C16H26N2O6S2, P21212) a = 19.171(2) Å, b = 21.116(2) Å, c = 5.0045(4) Å; (C18H30N2O6S2, P212121) a = 5.022(1) Å, b = 17.725(3) Å, c = 25.596(2) Å; and (C20H34N2O6S2, C2) a = 40.698(15) Å, b = 5.083(3) Å, c = 12.105(5) Å, β = 99.66(3)°.

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