Cell-specific translational regulation of S-adenosylmethionine decarboxylase mRNA. Influence of the structure of the 5' transcript leader on regulation by the upstream open reading frame.

A small upstream open reading frame (uORF), located 14 nucleotides from the cap in the 5' transcript leader (5' TL) of the mRNA encoding S-adenosylmethionine decarboxylase (AdoMetDC), suppresses translation of the downstream cistron in normal T cells and T cell lines. In the present study, we examined the structural features of the 5' TL that overcome this suppressive influence in cells of nonlymphoid origin. Initiation at the downstream cistron in nonlymphoid cells is by a cap-dependent mechanism that requires ribosome scanning along the 5' TL and does not involve an internal ribosome entry site. Extending the uORF so that it overlapped the major cistron by 101 nucleotides had no effect on translation of the downstream cistron in either HeLa or Jurkat cells. When the distance between the uORF and the cap was extended to 47 nucleotides, using sequence previously found to be neutral, translation of the major cistron was inhibited 5-fold in HeLa cells and the mRNA was moved from polysomes to monosomes, a location identical to that of the wild type mRNA in Jurkat cells. Therefore, in contrast to T cells, initiation at the uORF seems to be relatively infrequent in non-lymphoid cells due to its proximity to the cap, allowing efficient translation of the downstream cistron.

Structure and developmental expression of the chicken NGF receptor.

The nucleotide and deduced amino acid sequence of a cDNA clone of the chicken NGF receptor (NGFR) is reported and is compared with sequences of mammalian NGF receptors. A model is presented in which monodentate or bidentate binding of NGF dimers to repeated cysteine-rich sequence elements of the receptor yields low- or high-affinity NGF binding, respectively. In situ hybridization is used to characterize expression of NGFR in developing chick from 40 hr to 10 days of embryogenesis. NGFR mRNA expression is detected in premigratory neural crest cells, in epibranchial placode cells, and in all sensory, sympathetic and parasympathetic derivatives of these structures. In the embryonic CNS, NGFR mRNA is detected in the mantle zone but not the periventricular germinal zone throughout most of the neural tube. By Embryonic Day 8, NGFR mRNA is detected in a substantial fraction of cells in every brain region, with highest levels present in developing motor neurons. NGFR mRNA also is transiently expressed in many mesenchymal cell populations including cells in branchial arch, sclerotome, muscle anlagen, and feather follicles. The functional significance of wide-spread embryonic expression of the NGF receptor is discussed.

Biophysical and serologic comparison of four equine adenovirus isolates.

Four equine adenovirus isolates have been characterized with regard to their biophysical and serologic properties. Electron microscopic studies demonstrated that purified virions have a typical adenovirus morphologic characteristic, with 50-nm-long fiber projections at each vertex of an 80-nm-diameter icosahedron. Extracted viral DNA was found to be a linear duplex of molecular weight 21 to 22 x 10(6). All four isolates were found to have a buoyant density in CsCl of 1.346 +/- 0.002 g/ml. Hexon structural components prepared from each isolate were shown to carry the same relative net charge, as judged from anion exchange elution profiles. Sodium dodecyl sulfate (sodium lauryl sulfate)-polyacrylamide gel electrophoresis revealed that the four isolates were composed of which the electrophoretic migration pattern was distinct from that of a human adenovirus reference. Serologic data (serum-neutralization and hemagglutination-inhibition tests) did not reveal any distinct antigenic diversity among the four isolates. On the basis of data obtained in this study, it is proposed that equine adenovirus isolates thus far available do, in fact, constitute a single serotype.