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The dorsal blastopore lip of the early Xenopus laevis gastrula can organize a complete secondary body axis when transplanted to another embryo. A search for potential gene regulatory components specifically expressed in the organizer was undertaken that resulted in the identification of four types of complementary DNAs from homeobox-containing genes that fulfill this criterion. The most abundant of these encodes a DNA-binding specificity similar to that of the Drosophila melanogaster anterior morphogen bicoid. The other three are also homologous to developmentally significant Drosophila genes. These four genes may participate in the regulation of the developmental potential of the organizer.
The anteroposterior character of mesoderm induced by a peptide growth factor (XTC-MIF) was tested by transplantation into host Xenopus gastrulae. Both retinoic acid and a homeodomain protein were able to override the anteriorizing effect of the growth factor. Microinjection of a posteriorly expressed homeobox mRNA can respecify anteroposterior identity, transforming head mesoderm into tail-inducing mesoderm. Unexpectedly, overexpression of XIHbox 6 protein in the transplanted cells, without addition of growth factors, caused the formation of tail-like structures. The cells overexpressing XIHbox 6 were able to recruit cells from the host into the secondary axis. The results suggest that vertebrate homeodomain proteins are part of the biochemical pathway leading to the generation of the body axis.
Establishment of mesodermal tissues in the amphibian body involves a series of inductive interactions probably elicited by a variety of peptide growth factors. Results reported here suggest that mesodermal patterning involves an array of signalling molecules including DVR-4, a TGF-beta-like molecule. We show that ectopic expression of DVR-4 causes embryos to develop with an overall posterior and/or ventral character, and that DVR-4 induces ventral types of mesoderm in animal cap explants. Moreover, DVR-4 overrides the dorsalizing effects of activin. DVR-4 is therefore the first molecule reported both to induce posteroventral mesoderm and to counteract dorsalizing signals such as activin. Possible interactions between these molecules resulting in establishment of the embryonic body plan are discussed.
A novel protein tyrosine kinase (PTK) substrate, p120, has been previously implicated in ligand-induced signaling through the epidermal growth factor, platelet-derived growth factor and colony-stimulating factor 1 receptors, and in cell transformation by p60v-src. We have isolated a near full-length cDNA encoding murine p120. The encoded protein lacks significant homology with any reported protein, but it contains four copies of an imperfect 42 amino acid repeat that occurs 12.5 times in the protein encoded by Drosophila armadillo (arm), and its direct homologs, human plakoglobin (plak) and Xenopus laevis beta-catenin (beta-cat). The presence of this motif implies that p120 may share at least one aspect of its function with the arm protein and its homologs.
The virus-associated (VA) RNAI gene in human adenovirus 2 DNA has been shown by Wu (Wu, G. J. (1978) Proc. Natl. Acad. Sci. U. S. A. 75, 2175--2179) to be transcribed by RNA polymerase III in a human KB cell-free extract. In the present report we have examined the fidelity of transcription of adenovirus 2 DNA and Xenopus oocyte 5 S DNA templates by RNA polymerase III in extracts derived from cultured human, murine, and amphibian kidney cells, Size and sequences analysis of the discrete transcripts synthesized in these homologous and heterologous systems indicate that they result from accurate transcription of the corresponding genes. The specific transcripts identified include both the adenovirus VA RNAI and VA RNAII, Xenopus 5 S RNA, and VA RNAI and 5 S RNA species with elongated 3' termini. The extracts derived from the various cell types differ in the ability to discriminate between the two VA RNA genes or between the heterogeneous 5 S RNA genes in the cloned DNA fragment. Wherease the human cell extracts transcribe the VA RNAI and VA RNAII genes of adenovirus at a relative frequency close to that observed in isolated nuclei, the amphibian cell extract appears to transcribe only the VA RNAI gene. The amphibian cell extract transcribes primarily that 5 S RNA gene (within 5 S DNA) which encodes the dominant oocyte 5 S RNA, whereas the human cell extract transcribes at least two distinct 5 S RNA genes. Additionally, it is shown that the VA RNAI and VA RNAII genes have separate promotor sites. The kinetics of the transcription reactions have been examined and conditions optimal for specific transcription have been established by examining the effects of salt, metal ion, and template concentrations on both total and specific RNA synthesis. It is also shown that components in the cell-free extract (from human cells) are active in directing the accurate transcription of adenovirus DNA by purified RNA polymerase III.