A cytoplasmic determinant for dorsal axis formation in an early embryo of Xenopus laevis

In Xenopus laevis, dorsal cells that arise at the future dorsal side of an early cleaving embryo have already acquired the ability to cause axis formation. Since the distribution of cytoplasmic components is markedly heterogeneous in an egg and embryo, it has been supposed that the dorsal cells are endowed with the activity to form axial structures by inheriting a unique cytoplasmic component or components localized in the dorsal region of an egg or embryo. However, there has been no direct evidence for this. To examine the activity of the cytoplasm of dorsal cells, we injected cytoplasm (dorsal cytoplasm) from dorsal vegetal cells of a Xenopus 16-cell embryo into ventral vegetal cells of a simultaneous recipient. The cytoplasm caused secondary axis formation in 42% of recipients. Histological examination revealed that well-developed secondary axes included notochord, as well as a neural tube and somites. However, injection of cytoplasm of ventral vegetal cells never caused secondary axis and most recipients became normal tailbud embryos. Furthermore, about two-thirds of ventral isolated halves injected with dorsal cytoplasm formed axial structures. These results show that dorsal, but not ventral, cytoplasm contains the component or components responsible for axis formation. This can be the first step towards identifying the molecular basis of dorsal axis formation.

Association of an integral membrane protein with glucose transport and with anion transport

A monoclonal antibody that recognizes a cell-surface glycoprotein associated with glucose transport was reported previously. Additional information about the function and intracellular distribution of the antigen recognized by this antibody is presented. The monoclonal antibody recognizes a cell-surface and a cytoplasmic determinant. The density of the cell-surface determinant is heterogeneous within the cell population. The subpopulation of cells that carry the cell-surface determinant at high density correspond with a subset of cells that incorporate 2-deoxy-D-[3H]glucose more rapidly than the population as a whole. The monoclonal antibody is used, with cell-affinity chromatography, to isolate this subset of cells. The cytoplasmic determinant, to which the antibody binds, is associated with the cytoplasmic microfilaments but the antibody is not absorbed by actin. The cell-surface and cytoplasmic components are not identical since the apparent affinity of the antibody for each site is different. The portion of the antigen in the membrane behaves as an integral membrane protein while the remainder is tightly associated with the detergent-insoluble cytoskeleton. The expression of the antigen on the cell surface is modified by covalent attachment of an inhibitor of anion transport, 4,4′-diisothiocyano-2,2′-disulphonic stilbene. The possible interaction of the anion/lactate transporter with the glucose transporter is discussed.

Quantitative regulation of acetylcholinesterase development in the muscle lineage cells of cleavage-arrested ascidian embryos

Some embryos of Ciona intestinalis which were permanently cleavage-arrested with cytochalasin B at the 1-cell, 4-cell, or 8-cell stages produced, after 12 or 16 h of development time (18 °C), a level of muscle acetylcholinesterase activity equal to that found in normal early and later larval stage embryos of the same age. Enzyme activity was measured quantitatively in single whole embryos by a colorimetric procedure using microdensitometry. Quantitative regulation of a differentiation end product indicated that the usual transcriptional and translational control mechanisms for that histospecific protein continued to operate normally in the cleavage-arrested embryos. Acetylcholinesterase expression was apparently regulated independently of the usual cell cytoplasmic volume in the muscle lineage cells and possibly also independently of the normal nuclear number in the lineage. There is an egg cytoplasmic determinant that is segregated into the muscle lineage cells during cleavage and which appears to specify the pathway of larval muscle development. Quantitative control of muscle acetylcholinesterase is possibly one of the consequences of how the agent releases genetic expression in the presumptive muscle cells. Quantitative regulation was not, however, a general functional activity of cleavage-arrested embryos. Mitochondrial cytochrome oxidase, an enzyme whose development is believed to be unaffected by cytoplasmic determinants, was not regulated quantitatively in cleavage-arrested embryos. Cytochrome oxidase activity of cleavage-arrested embryos, measured in single whole embryos by a colorimetric microdensitometry assay, increased only slightly during 16 h of development time whereas the activity in normal control embryos doubled during that time.

ULTRAVIOLET MUTAGENESIS STUDIES OF [psi], A CYTOPLASMIC DETERMINANT OF SACCHAROMYCES CEREVISIAE

ABSTRACT UV mutagenesis was used to probe the molecular nature of [psi], a nonmitochondrial cytoplasmic determinant of Saccharomyces cereuisiae involved in the control of nonsense suppression. The UV-induced mutation from [psi +] to [psi -] showed characteristics of forward nuclear gene mutation in terms of frequency, induction kinetics, occurrence of whole and sectored mutant clones and the effect of the stage in the growth cycle on mutation frequency. The involvement of pyrimidine dimers in the premutational lesion giving the [psi -] mutation was demonstrated by photoreactivation. UV-induced damage to the [psi] genetic determinant was shown to be repaired by nuclear-coded repair enzymes that are responsible for the repair of nuclear DNA damage. UV-induced damage to mitochondrial DNA appeared to be, at least partly, under the control of different repair processes. The evidence obtained suggests that the [psi] determinant is DNA.