Genes of the Polycomb group (PcG) of Drosophila encode proteins necessary for the maintenance of transcriptional repression of homeotic genes. PcG proteins are thought to act by binding as multiprotein complexes to DNA through Polycomb group response elements (PREs); however, specific DNA binding has not been demonstrated for any of the PcG proteins. We have identified a sequence-specific DNA binding protein that interacts with a PRE from the Drosophila engrailed gene. This protein (PHO) is a homolog of the ubiquitous mammalian transcription factor Yin Yang-1 and is encoded by pleiohomeotic, a known member of the PcG. We propose that PHO acts to anchor PcG protein complexes to DNA.

Accumulating evidence indicates that a common set of genes and mechanisms regulates the developmental processes of a variety of triploblastic organisms despite large variation in their body plans. To what extent these same genes and mechanisms are also conserved among diploblasts, which arose earlier in metazoan evolution, is unclear. We have characterized a hydra homologue of the fork head/HNF-3 class of winged-helix proteins, termed budhead, whose expression patterns suggest a role(s) similar to that found in vertebrates. The vertebrate HNF-3 beta homologues are expressed early in embryogenesis in regions that have organizer properties, and later they have several roles, among them an important role in rostral head formation. In the adult hydra, where axial patterning processes are continuously active, budhead is expressed in the upper part of the head, which has organizer properties. It is also expressed during the formation of a new axis as part of the development of a bud, hydra's asexual form of reproduction. Expression during later stages of budding, during head regeneration and the formation of ectopic heads, indicates a role in head formation. It is likely that budhead plays a critical role in head as well as axis formation in hydra.

Reactions of hydroxyl radicals with DNA form a variety of base and sugar products and 8,5'-cyclopurine 2'-deoxyribonucleoside residues in DNA. Here we report the effect of DNA conformation on the yields of 8,5'-cyclopurine 2'-deoxynucleosides and the ratios of their (5'R)- and (5'S)-diastereomers. Calf thymus DNA in native (double-stranded DNA) or heat-denatured form (single-stranded DNA) was exposed to hydroxyl radicals generated by ionizing radiation in nitrous oxide-saturated phosphate buffer. Doses ranging from 10 to 40 Gy were used to ensure low levels of damage to DNA and thus to preserve its secondary structure in experiments with double-stranded DNA (ds-DNA). After irradiation, DNA was hydrolysed enzymatically to 2'-deoxyribonucleosides. The hydrolysates were dried, trimethylsilylated, and analyzed by capillary gas chromatography-mass spectrometry with selected-ion monitoring. An internal standard was used for quantitative measurements and added to DNA samples prior to enzymatic hydrolysis. The yields of 8,5'-cyclo-2'-deoxyadenosine and 8,5'-cyclo-2'-deoxyguanosine in single-stranded DNA (ss-DNA) were higher than those in ds-DNA. The (5'R)-diastereomers of both compounds were found to predominate over their (5'S)-diastereomers in ss-DNA. In contrast, the yields of the (5'S)-diastereomers in ds-DNA were slightly higher than those of the (5'R)-diastereomers. The G values of 8,5'-cyclo-2'-deoxyadenosine in ss-DNA and ds-DNA were 0.042 and 0.025, respectively. Those of 8,5'-cyclo-2'-deoxyguanosine in ss-DNA and ds-DNA were 0.038 and 0.017, respectively.

The "amber" (am) mutants of bacteriophage T4 are a recently discovered class of mutants that can replicate in certain derivatives of Escherichia coli strain K-12 but not in E. coli B. ' The bacterial strains that support growth of the am mutants, thus making their isolation and propagation possible, are called the "permissive" hosts. The biochemical basis of this permissiveness has not yet been clarified. With some of the am mutants, infection of E. coli B leads to a subnormal production of phage DNA; with others, DNA is formed but infection is abortive because of failure of late stages of phage development.2It is known that infection of E. coli with a T-even bacteriophage leads to the appearance of several new enzyme activities and to an increase in several other enzyme activities, all related to synthesis of phage DNA ("early enzymes").312 These Downloaded by guest on

The formation of ventral mesoderm has been traditionally viewed as a result of a lack of dorsal signaling and therefore assumed to be a default state of mesodermal development. The discovery that bone morphogenetic protein 4 (BMP4) can induce ventral mesoderm led to the suggestion that the induction of the ventral mesoderm requires a different signaling pathway than the induction of the dorsal mesoderm. However, the individual components of this pathway remained largely unknown. Here we report the identification of a novel Xenopus homeobox gene PV.1 (posterior-ventral 1) that is capable of mediating induction of ventral mesoderm. This gene is activated in blastula stage Xenopus embryos, its expression peaks during gastrulation and declines rapidly after neurulation is complete. PV.1 is expressed in the ventral marginal zone of blastulae and later in the posterior ventral area of gastrulae and neurulae. PV.1 is inducible in uncommited ectoderm by the ventralizing growth factor BMP4 and counteracts the dorsalizing effects of the dominant negative BMP4 receptor. Overexpression of PV.1 yields ventralized tadpoles and rescues embryos partially dorsalized by LiCl treatment. In animal caps, PV.1 ventralizes induction by activin and inhibits expression of dorsal specific genes. All of these effects mimic those previously reported for BMP4. These observations suggest that PV.1 is a critical component in the formation of ventral mesoderm and possibly mediates the effects of BMP4.The dorsal-ventral patterning of mesoderm is central to the specification of the vertebrate body axis. In Xenopus, cytoplasmically localized information and early embryonic inductions play important roles in this process. Dorsal-ventral polarity is established shortly after fertilization, when the cortex of the Xenopus egg rotates by about 30 degrees relative to the cytoplasm, thereby localizing important dorsal determinants (1, 2). As early as the 32-cell stage, the initial induction of mesoderm occurs with signals emanating from dorsal vegetal cells inducing dorsal mesoderm and ventral vegetal cells inducing ventral mesoderm (3, 4). Mesoderm is further patterned during gastrulation by dorsalizing signals arising from the Spemann organizer. Cells of the organizer induce the rest of the mesoderm in a graded fashion, regionalizing it into zones of somite, lateral plate, and blood islands (5).A great deal of effort has focused on identifying the extracellular molecules that constitute these inducing signals.Several members of the transforming growth factor-,B and fibroblast growth factor families are now clearly implicated as candidates for the putative dorsal and ventral mesoderm inducing signals, respectively (for review see ref. 6; refs. 7 and 8). Signaling proteins that modulate (rather than mediate) mesoderm induction, such as Xwnt 8 and ADMP-1, have also been identified (9, 10). Both of these factors have been proposed to be able to convert the fate of dorsal tissues to ventral. Recently, a potent dorsalizing factor (chordin), activat...