The first chordates appear in the fossil record at the time of the Cambrian explosion, nearly 550 million years ago. The modern ascidian tadpole represents a plausible approximation to these ancestral chordates. To illuminate the origins of chordate and vertebrates, we generated a draft of the protein-coding portion of the genome of the most studied ascidian, Ciona intestinalis. The Ciona genome contains ϳ16,000 protein-coding genes, similar to the number in other invertebrates, but only half that found in vertebrates. Vertebrate gene families are typically found in simplified form in Ciona, suggesting that ascidians contain the basic ancestral complement of genes involved in cell signaling and development. The ascidian genome has also acquired a number of lineage-specific innovations, including a group of genes engaged in cellulose metabolism that are related to those in bacteria and fungi.

ABSTRACTcDNAs representing the a subunit of polyomavirus enhancer binding protein 2 (PEBP2; also cafled PEA2) were isolated. The products of the cDNAs are highly homologous to that of Drosophila segmentation gene runt (run) for an N-proximal 128-amino acid region showing 66% identity. The run homology region encompasses the domain capable of binding to a specific nucleotide sequence motif and of dimerizing with the companion 13 subunit. The human AMLI gene related to t(8;21) acute myeloid leukemia also has a run homology region. Together with the 18 subunit, which increases the affinity of the a subunit to DNA without binding to DNA by itself, PEBP2 represents a newly discovered family of transcription factor. The major species of PEBP2a mRNA was expressed in T-cell lines but not in B-cell lines tested. Evidence indicated that PEBP2 functions as a transcriptional activator and is,involved in regulation of T-ceil-specific gene expression.Potential use of polyomavirus (Py) as a probe of mouse development has been recognized since it was observed that the wild-type Py did not grow in embryonal carcinoma cells but that it grew when the cells were induced to differentiate (1)(2)(3)(4)(5). Through the analysis of the Py enhancer, which determines the differentiation-dependent and cell-typespecific expression of the viral genes, a number of transcription factors involved in developmental regulation have been identified (6-10).PEBP2/PEA2, which binds to both the A and B cores of the Py enhancer (11), is undetectable in embryonal carcinoma F9 cells and becomes detectable after the cells are induced to differentiate (6,7). It specifically recognizes a consensus sequence, R/TACCRCA (R, purine), which was originally identified in the Py enhancer (11) and is also compatible with the core motif of murine leukemia virus enhancers (12,13). In addition, many T-cell-specific genes, such as T-cell receptor (TCR) a, X3, yand 8 and CD3e genes, contain potential PEBP2 binding sites, suggesting the possibility that PEBP2 is involved in T-cell-specific gene expression (13 [708][709][710][711][712][713][714]. The mutations are the same as the M2 mutation (14). For expression in mammalian cells, the coding regions of al, a2, and 831 cDNAs were cloned into the Xho I site of pCDMPy, in which the Py origin and enhancer region were deleted from pCDM8 (Invitrogen), resulting in pCDMPy-al, pCDMPy-a2, and pCDMPy-31. The bacterial expression plasmids pETal and pETa2, carrying the whole coding regions of al and a2 cDNAs, were constructed in the same way as pETf82 (16). To produce deletion mutants of a2, the whole coding region of a2 cDNA was inserted into the BamHI/HindIII fragment of plasmid pQE9 (Qiagen) resulting in pQE9-a2 encoding a fusion protein tagged with an N-terminal histidine cluster. Various deletions were introduced into pQE9-a2 using appropriate restriction sites: N94C306 encodes a region of aa 94-306; N1C226 encodes aa 1-226; N1C158 encodes aa 1-158; N80C226 encodes aa 80-226. N1C226, N1C158, and N80C226 proteins have one, two, o...

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Cephalochordates, urochordates, and vertebrates evolved from a common ancestor over 520 million years ago. To improve our understanding of chordate evolution and the origin of vertebrates, we intensively searched for particular genes, gene families, and conserved noncoding elements in the sequenced genome of the cephalochordate Branchiostoma floridae, commonly called amphioxus or lancelets. Special attention was given to homeobox genes, opsin genes, genes involved in neural crest development, nuclear receptor genes, genes encoding components of the endocrine and immune systems, and conserved cis-regulatory enhancers. The amphioxus genome contains a basic set of chordate genes involved in development and cell signaling, including a fifteenth Hox gene. This set includes many genes that were co-opted in vertebrates for new roles in neural crest development and adaptive immunity. However, where amphioxus has a single gene, vertebrates often have two, three, or four paralogs derived from two whole-genome duplication events. In addition, several transcriptional enhancers are conserved between amphioxus and vertebrates-a very wide phylogenetic distance. In contrast, urochordate genomes have lost many genes, including a diversity of homeobox families and genes involved in steroid hormone function. The amphioxus genome also exhibits derived features, including duplications of opsins and genes proposed to function in innate immunity and endocrine systems. Our results indicate that the amphioxus genome is elemental to an understanding of the biology and evolution of nonchordate deuterostomes, invertebrate chordates, and vertebrates.

Angiogenesis is an important event for embryonic organogenesis as well as for tissue repair in the adult. Here, we show that hematopoietic stem cells (HSCs) play important roles for angiogenesis during embryogenesis. To investigate the role of HSCs in endothelial cell (EC) development, we analyzed AML1-deficient embryos, which lack definitive hematopoiesis. These embryos showed defective angiogenesis in the head and pericardium. Para-aortic splanchnopleural (P-Sp) explant cultures on stromal cells (P-Sp culture) did not generate definitive hematopoietic cells and showed defective angiogenesis in the AML1 null embryo. Disrupted angiogenesis in P-Sp cultures from AML1 null embryos was rescued by addition of HSCs or angiopoietin-1 (Ang1). HSCs, which express Ang1, directly promoted migration of ECs in vivo and in vitro. These results indicate that HSCs are critical for angiogenesis.