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Hemocyanins are large multi-subunit copper proteins that transport oxygen in many arthropods and molluscs. Comparison of the amino acid sequence data for seven different subunits of arthropod hemocyanins from crustaceans and chelicerates shows many highly conserved residues and extensive regions of near identity. This correspondence can be matched closely with the three domain structure established by x-ray crystallography for spiny lobster hemocyanin. The degree of identity is particularly striking in the second domain of the subunit that contains the six histidines which ligate the two oxygen-binding copper atoms. The polypeptide architecture of spiny lobster hemocyanin appears to be the same in all arthropods. This structure must therefore be at least as old as the estimated time of divergence of crustaceans and chelicerates, about 540 to 600 million years ago.

The primary structure of an extracellular ribonuclease (RNase LE) from Pi-depleted media of cultured cells of Lycopersicon esculenturn L. cv. Lukullus has been determined. This was carried out by analysis of peptides isolated after enzymatic and chemical cleavage of the reduced and S-ethylpyridylated protein.RNase LE consists of 205 amino acid residues and has a molecular mass of 22 666 Da and an isoelectric point of 4.24. The enzyme contains 10 half-cystines. There are no potential N-glycosylation sites in the sequence.The sequence of RNase LE is homologous with those of self-incompatibility proteins of several higher plant species and with those of a number of fungal RNases. The sequence similarity with the family of self-incompatility proteins is greater than with the fungal RNases, suggesting that the self-incompatibility proteins arose from ancestral RNase by gene duplication after the divergence of higher plants and fungi. Two pentapeptide sequences, i. e. HGLWP and KHGTC (or KHGSC), are present at identical positions in all the aligned proteins, suggesting that they contribute to the active site.RNA metabolism, which includes processing, turnover and degradation of cellular RNA, has emerged as a complex and important determinant of gene expression in living cells. Much of our current knowledge on enzymes hydrolyzing RNA (RNases) and their in vivo functions comes from studies with Escherichia coli [l]. With respect to structural characterization and regulation of RNases from plant origin, knowledge lags far behind that already well established for the enzymes from fungi [2] and mammals [3].A number of physiological functions in plants were found to be associated with changes in nucleolytic activities (see [4] and [5] for reviews). More recent studies on nucleolytic and ribonucleolytic enzymes have shown that their expression is regulated by hormones [6], light and senescence [7], plant development [8], aging and wounding [9] or nutrient starvation [lo, 111 (see below). Investigation of these enzymes may lead to the identification of regulatory elements and signals for plant gene expression in that field.

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The primary structure of subunit b of Panulirus interruptus hemocyanin has been derived from two digests (trypsin and CNBr) and, in some cases, with aid from the similarity with the sequence of subunit a. Differences between the amidation states of Asx and Glx residues in subunit b relative to a were investigated more thoroughly. When compared to the sequence of subunit a, 18 differences (2.7%) were found and certain heterogeneities, indicating the presence of a minor subunit b', were observed.Several differences in properties between subunits a and b, including their anomalous behaviour on SDS/ polyacrylamide gel electrophoresis, could be explained by amino acid replacements.Hemocyanins are large multi-subunit copper-containing oxygen carriers occurring freely dissolved in the hemolymph of many molluscs and arthropods. Arthropod hemocyanins are composed of hexamers or multi-hexamers with subunits of about 75 kDa, each of which contains one binuclear copper site 11, 21. As in all arthropods, hemocyanin of the spiny lobster Punulirus interruptus displays subunit heterogeneity Both X-ray diffraction and amino acid sequence studies have also been performed. Subunits a and b, accounting for 80 -90% of the monomers upon dissociation of the hexamers, are very similar, but subunit c is quite different [3, 6-81. The complete amino acid sequence of subunit a has been published recently [9]. The three-dimensional structure has been determined at 0.32-nm resolution [4, 101. Since the latter study was performed with crystals containing a mixture of subunits a and b in roughly equal amounts [4, lo], knowledge of the amino acid sequence of subunit b is essential for a reliable interpretation of the electron-density map. Furthermore, comparison of the primary structures of subunit a and b may explain the difference in their apparent molecular masses on SDS/polyacrylamide gels.