Recognition of specific DNA sequences in eukaryotic chromosomes.

The packaging of DNA into chromatin probably places certain restrictions on how specific DNA sequences can be recognized by DNA sequence specific recognition proteins (SRP). Several unique features of this type of interaction are discussed. Specifically, as a consequence of the coiling of the DNA about a histone core, it is proposed that DNA recognition sites will be compound and that each element of the compound recognition site will be about 10 - 20 b.p. in length and distributed at approximately 80 b.p. intervals--the periodicity of the DNA wrapping around the nucleosome.     

Progressive determination during formation of the anteroposterior axis in Xenopus laevis

The cement gland is an ectodermal organ in the head of frog embryos, lying anterior to any neural tissue. As analyzed by specific RNA expression, cement gland, like neural tissue, was induced by the dorsal mesoderm. Interestingly, mesoderm with the highest cement gland-inducing potential lay posterior to the ectoderm fated to form this organ, indicating that its induction occurred at a distance from the inducer source. Cement gland induction first occurred during early gastrulation. However, most initially induced cells did not contribute to the mature cement gland, but instead formed part of the neural plate. This change in fate could be reconstituted in vitro. These results suggest that determination of part of the anteroposterior axis occurs progressively, where future neural ectoderm is first induced to a cement glandlike state. As gastrulation proceeds, further induction by mesoderm may override this state, which persists only in the extreme anterior of the embryo.     

Prostaglandin E2 induces receptive behaviors in female Xenopus laevis

The object of this study was to examine the effects of exogenous and endogenous prostaglandin E2 (PGE2) on the sexual behavior of female South African clawed frogs, Xenopus laevis. Ticking and leg extension, which communicate sexual unreceptivity to males, were studied in intact, ovariectomized, and ovariectomized-oviductectomized females. The onset of the PGE2 behavioral effect occurs within 30 sec to 3 min of injection for intact and ovariectomized females; for ovariectomized-oviductectomized females, the latency period for the effect ranges from 10-20 min. PGE2 induced receptivity in doses as low as 0.03 microgram/frog. Injection of the prostaglandin synthesis inhibitors, indomethacin and flurbiprofen (FBP), blocked chorionic gonadotropin- (HCG-) induced behavioral receptivity, suggesting that endogenous prostaglandin synthesis may have a role in regulating female sexual behavior. Flurbiprofen blockade of HCG-induced receptivity was reversed by PGE2 administration, suggesting that FBP's effects are PG synthesis-specific.     

THE PROTEIN COMPOSITION OF NUCLEAR CRYSTALS IN LEAF CELLS

The chemical composition of perinuclear crystals of Lychnis chalcedonica and intranuclear crystals of Dianthus barbatus in leaf tissue embedded in glycol methacrylate was tested by differential enzyme digestion of ultra-thin sections. The Lychnis crystals were digested readily by pepsin but with great difficulty and never to completion by trypsin. The Dianthus crystals were rapidly and completely digested by pronase; inconsistently and with difficulty by pepsin; and were unaffected by trypsin. Pre-incubation with water increased the effectiveness of pepsin digestion. Neither RNase nor DNase had any effect, either alone, or when preceded or followed by the proteinases. It was concluded that the Lychnis crystals are composed of protein that is probably high in aromatic amino acid content, and possibly low in the basic amino acids arginine and lysine. The Dianthus crystals also seem to be protein, but because of their amino acid composition or conformation, and possibly because of complex reactions with the fixative, these crystals are not readily hydrolyzed by pepsin and trypsin.