The Spemann organizer in amphibian embryos is a tissue with potent head-inducing activity, the molecular nature of which is unresolved. Here we describe dickkopf-1 (dkk-1), which encodes Dkk-1, a secreted inducer of Spemann's organizer in Xenopus and a member of a new protein family. Injections of mRNA and antibody indicate that dkk-1 is sufficient and necessary to cause head induction. dkk-1 s a potent antagonist of Wnt signalling, suggesting that dkk genes encode a family of secreted Wnt inhibitors.

The Wnt family of secreted glycoproteins mediate cell cell interactions during cell growth and differentiation in both embryos and adults. Canonical Wnt signalling by way of the beta-catenin pathway is transduced by two receptor families. Frizzled proteins and lipoprotein-receptor-related proteins 5 and 6 (LRP5/6) bind Wnts and transmit their signal by stabilizing intracellular beta-catenin. Wnt/beta-catenin signalling is inhibited by the secreted protein Dickkopf1 (Dkk1), a member of a multigene family, which induces head formation in amphibian embryos. Dkk1 has been shown to inhibit Wnt signalling by binding to and antagonizing LRP5/6. Here we show that the transmembrane proteins Kremen1 and Kremen2 are high-affinity Dkk1 receptors that functionally cooperate with Dkk1 to block Wnt/beta-catenin signalling. Kremen2 forms a ternary complex with Dkk1 and LRP6, and induces rapid endocytosis and removal of the Wnt receptor LRP6 from the plasma membrane. The results indicate that Kremen1 and Kremen2 are components of a membrane complex modulating canonical Wnt signalling through LRP6 in vertebrates.

Members of the transforming growth factor-beta (TGF-beta) superfamily, including TGF-beta, bone morphogenetic proteins (BMPs), activins and nodals, are vital for regulating growth and differentiation. These growth factors transduce their signals through pairs of transmembrane type I and type II receptor kinases. Here, we have cloned a transmembrane protein, BAMBI, which is related to TGF-beta-family type I receptors but lacks an intracellular kinase domain. We show that BAMBI is co-expressed with the ventralizing morphogen BMP4 (refs 5, 6) during Xenopus embryogenesis and that it requires BMP signalling for its expression. The protein stably associates with TGF-beta-family receptors and inhibits BMP and activin as well as TGF-beta signalling. Finally, we provide evidence that BAMBI's inhibitory effects are mediated by its intracellular domain, which resembles the homodimerization interface of a type I receptor and prevents the formation of receptor complexes. The results indicate that BAMBI negatively regulates TGF-beta-family signalling by a regulatory mechanism involving the interaction of signalling receptors with a pseudoreceptor.

Genomic clones coding for human fibroblast collagenase were isolated. By constructing and transfecting mutants with 5' and 3' deletion mutations of the 5' control region of the gene into human or murine cells, we delimited a 32-base-pair sequence at positions -73 to -42 which is required for the induction of transcription by the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate. The DNA element behaves as a 12-0-tetradecanoyl-phorbol-13-acetate-inducible enhancer: it mediates the stimulation of transcription to the heterologous herpes simplex virus thymidine kinase promoter and acts in a position-and orientationindependent manner. Differences in enhancer efficiency in different cell lines are interpreted to indicate differences in the activity of a trans-acting factor.

The identification and taxonomy of papillomaviruses has become increasingly complex, as approximately 70 human papillomavirus (HPV) types have been described and novel HPV genomes continue to be identified. Methods and corresponding DNA sequence data bases were designed for the reliable identification of mucosal HPV genomes from clinical specimens. HPVs are identified by the amplification of a fragment of the L1 region by consensus primer polymerase chain reaction (PCR) and subsequent hybridization or restriction fragment length polymorphism analysis. L1 PCR fragments may be further characterized by nucleotide sequencing. Conservation of 30 (of 151) predicted amino acids identifies HPV genomic fragments, and nucleotide sequence alignments allow calculation of their phylogenetic relatedness. Sequence differences > 10% from any known HPV type suggest a novel HPV type. Phylogenetic relationships with known HPV types may permit predictions of biology. With these criteria, 10 PCR fragments were identified that would qualify as new genital HPV types after complete genomic isolation.

Positive‐stranded genomic RNA of coronavirus MHV and its six subgenomic mRNAs are synthesized in the cytoplasm of the host cell. The mRNAs are composed of leader and body sequences which are non‐contiguous on the genome and are fused together in the cytoplasm by a mechanism which appears to involve an unusual and specific ‘polymerase jumping’ event.

The genes coding for the three membrane polypeptides of Semliki Forest virus have been sequenced and the primary structures of the proteins deduced. The amino acid sequence gives further insight into how the transmembrane structure of the three-chain virus membrane glycoprotein is generated in the infected cell.

A DNA-unwinding protein has been purified to homogeneity from E. coli. This protein has a molecular weight of about 22,000, as judged by its electrophoretic mobility on polyacrylamide gels containing sodium dodecylsulfate, and it appears to be present in about 800 copies per log-phase cell. It binds tightly and cooperatively to single-stranded DNA, and much less tightly, if at all, to RNA or double-stranded DNA.Like the T4 gene-32 protein characterized previously, the E. coli DNA-unwinding protein depresses the melting temperature of double-stranded DNAs, with regions rich in A-T base-pairs being preferentially melted. The E. coli protein strongly stimulates in vitro DNA synthesis by E. coli DNA polymerase II on appropriate templates; however, no stimulation is found with purified polymerases I or III of E. coli, or with T4 DNA polymerase. In contrast, gene-32 protein stimulates only the T4 DNA polymerase in a parallel assay.In vitro studies with purified DNA reveal that, at ionic strengths and temperatures in the physiological range, the double-helical DNA conformation is overwhelmingly stable relative to unpaired single-strands (1). In vivo, both DNA replication and genetic recombination would seem to demand a considerable loosening of this helical structure.Recent work suggests that such DNA unwinding is facilitated within a cell by the presence of a special type of protein that drastically lowers helix stability by virtue of its tight cooperative binding to exposed single strands of DNA (2). The first protein of this type to be characterized was found in Escherichia coli infected with T4 bacteriophage, and is the product of T4 gene-32 (2-5). This "DNA-unwinding" protein is required for both genetic recombination (6, 7) and replication (8, 9) DNA from bacteriophages lambda and T7 was prepared by phenol extractions of the purified phage. Exonuclease 1Il-treated DNA template (from phage T7) was prepared by incubation of 1 jmol of DNA (as nucleotide) with 2500 units of exonuclease III for 5 min at 300 in 4.5 ml of a buffer containing 0.09 M Tris-acetate (pH 8.2)-2 mM 2-mercaptoethanol-2 mM MgCl2. The reaction was terminated by heating for 5 min at 650. As judged by the maximum extent of DNA polymerase I-catalyzed repair at 300, about 500 nucleotides were digested per phage equivalent of DNA. Exonuclease III-treated, nicked DNA template from calfthymus was prepared as described (14); it contains many short single-stranded gaps, rather than the relatively long, single-stranded, termini expected for the T7 DNA.