Selective modification of proteins by ubiquitination is directed by diverse families of ubiquitin-protein ligases (or E3s). A large collection of E3s use Cullins (CULs) as scaffolds to form multisubunit E3 complexes in which the CUL binds a target recognition subcomplex and the RBX1 docking protein, which delivers the activated ubiquitin moiety. Arabidopsis and rice contain a large collection of CUL isoforms, indicating that multiple CUL-based E3s exist in plants. Here we show that Arabidopsis CUL3a and CUL3b associate with RBX1 and members of the broad complex/tramtrack/bric-a-brac (BTB) protein family to form BTB E3s. Eighty genes encoding BTB domain-containing proteins were identified in the Arabidopsis genome, indicating that a diverse array of BTB E3s is possible. In addition to the BTB domain, the encoded proteins also contain various other interaction motifs that likely serve as target recognition elements. DNA microarray analyses show that BTB genes are expressed widely in the plant and that tissue-specific and isoform-specific patterns exist. Arabidopsis defective in both CUL3a and CUL3b are embryo-lethal, indicating that BTB E3s are essential for plant development.

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We injected chick syncytial strain of reticuloendotheliosis virus (CS-REV) and wild type and recombinant avian leukosis virus (ALV) near the blastoderm of unincubated fertilized embryos and CS-REV intra-abdominally at day of hatch, and we progeny tested the surviving ALV viremic males and REV viremic males and females for transmitted viral genetic material. A number of positive progeny were identified and their deoxyribonucleic acid (DNA) analyzed for restriction enzyme fragments that hybridized with viral genetic material. Most of the progeny had simple restriction enzyme patterns unlike the viremic parents or congenitally infected progeny. This is suggestive evidence that retroviral genetic information has been inserted into the germ line of chickens.