While a large number of cydins have been described in animals and yeasts, very limited information is available regarding cydins in plants. We describe here the isolation of cDNA clones encoding four putative mitotic cyclins from maize. AU four cyclins were able to induce maturation of Xenopus oocytes, demonstrating that they can act as mitotic cyclins in this system. Northern analysis showed that all four cyclins were expressed only in actively dividing tissues and organs, with a stronger correlation between expression and mitotic activity than is observed with cdc2. The deduced protein sequences suggest that the four maize cyclins belong to the cyclin A and B families identified from animal and yeast studies but that they cannot be described easily as either A-type or B-type cyclins. However, comparison with previously cloned plant cyclins shows that cyclins in higher plants form three distinct structural groups that have been conserved in both monocotyledonous and dicotyledonous species and that cyclins from all three groups are present within a single plant species.Studies on the mechanism of cell division control in eukaryotic cells (1) have shown that protein kinases encoded by homologs of the Schizosaccharomyces pombe cdc2 gene (2), in association with proteins known as cyclins (3), play a key role in driving the cell cycle. Cyclins from the six structural types so far identified are presumed to form part of a kinase complex in association with p34cdc2 or closely related protein kinases of the cdk family (2, 4, 5). The large diversity of cyclins, in addition to that of their cdk counterparts, is believed to account for the specific phosphorylation of different sets of substrates at successive transitions of the cell cycle (2, 5).Significant differences exist between plants and animals in the regulation of cell division during development (6-9), but much less is known about cell cycle regulators in plants (8,9

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A cDNA encoding for a novel rainbow trout SHBG was identified and characterized. Phylogenetic analysis showed that this novel SHBG, named SHBGb, was a highly divergent paralog of the classical SHBG (SHBGa) form previously known in vertebrates including zebrafish, seabass, and rainbow trout. Using all available sequences, no SHBGb-like sequence could be identified in any fish species besides Atlantic salmon. Rainbow trout SHBGa and SHBGb share only 26% sequence identity at the amino acid level and exhibit totally distinct tissue distribution, thus demonstrating a functional shift of SHBGb. Indeed, shbga mRNA was predominantly expressed in liver and spleen but could not be detected in the ovary, whereas shbgb had a predominant ovarian expression but could not be detected in liver. Despite its high divergence, rainbow trout SHBGb expressed in COS-7 cells could bind estradiol and testosterone with high affinity and specificity. Both rainbow trout shbgb mRNA and proteins were localized to the granulosa cells of vitellogenic ovarian follicles, whereas SHBGb immunoreactivity was also found in theca cells. Finally, shbgb ovarian mRNA expression exhibited a significant drop between late vitellogenesis and oocyte maturation at a time when ovarian aromatase (cyp19a) gene expression and estradiol circulating levels exhibited a dramatic decrease. Together, these observations show that SHBGb is a functional and highly divergent SHBG paralog probably arising from a salmonid-specific duplication of the shbg gene.