Morphf mutants of Rous sarcoma virus: nucleotide sequencing analysis suggests that a class of morphf mutants was generated through splicing of a cryptic intron

The nature of the lesions involved in producing the fusiform phenotype of three mutants (WO101, WO201, and tsST529) of the Schmidt-Ruppin A strain of Rous sarcoma virus (RSV) was determined by molecular cloning and DNA sequencing. WO101 and WO201 contained an in-frame deletion of the v-src region coding for amino acids 116 to 140 of p60v-src. The deleted segment was flanked by consensus splice donor and acceptor sequences and contained an appropriately positioned branchpoint acceptor consensus sequence, suggesting that the deletion occurred through an aberrant RNA splicing event. S1 mapping experiments performed on RNA isolated from chicken cells infected with molecularly cloned wild-type RSV DNA suggested that the splice acceptor involved in the generation of this deletion was utilized at a low frequency (less than 1.0%) in wild-type RSV-infected cells. These results suggested that stable mutations may have arisen in the coding sequence of a eucaryotic viral transforming gene as a result of a probable aberrant RNA splicing event followed by reverse transcription into DNA. ST529 was found to harbor the same deletion present in WO101 and WO201 but also contained a point mutation which resulted in the substitution of lysine for glutamic acid at position 93. This change and the resulting large change in local charge were presumably required for the temperature-sensitive transformation phenotype of ST529. These results, together with other known deletions that produce fusiform mutants, suggested that a region within the amino-terminal one-third coding region of the src gene contributed to a structural domain of p60v-src that was important for controlling some morphological parameters of transformation in cells infected with RSV.

DNA sequence encoding the amino-terminal region of the human c-src protein: implications of sequence divergence among src-type kinase oncogenes

We sequenced the 5'-coding region of the human c-src gene, exons 2 through 5, corresponding to one-third of the human c-src protein consisting of 536 amino acids. Sequence analysis of the src type of protein kinases revealed that the amino-terminal region encoded by exon 2 contains sequences specific for the src proteins and raised the possibility that this region is involved in the recognition of a src-specific substrate(s) or receptor(s).

Expression of Ia antigens by murine kidney epithelium after exposure to streptozotocin

In the normal murine kidney, Ia antigens are expressed by dendritic cells located within the interstitial connective tissue and scattered cells within the glomerulus. After receiving multiple low doses of streptozotocin, a nitrosourea derivative of glucose, kidney epithelium labeled intensely with anti-Ia antibodies. Ultrastructural immunohistochemistry indicated that the epithelial cells of the proximal convoluted tubules expressed Ia antigens on their basolateral surfaces while remaining Ia- on their luminal surfaces. This response to streptozotocin does not appear to be related to the diabetogenic potential of the drug, because BALB/cJ mice, which remain normoglycemic after treatment with streptozotocin, also exhibited strongly Ia+ tubular epithelium after treatment with streptozotocin.

Isolation of murine thymic epithelium and an improved method for its propagation in vitro

A reliable and reproducible method for the isolation and propagation of thymic epithelial cells is described. Thymic epithelial cells from enzymatically dissociated thymus stroma are first enriched by separation on a discontinuous Percoll density gradient. The cell fractions enriched for epithelial cells are then cultured with irradiated fibroblasts in Ham's F-12 nutrient medium. Colonies of cells in these cultures contain keratin and exhibit morphologic characteristics of epithelial cells. When subcultured, the epithelial cells no longer require irradiated fibroblasts as filler cells. Some of the epithelial cells in vitro retain expression of class II (Ia) major histocompatibility antigens. The generation of defined cultures of thymic epithelial cells promises to be useful in defining their role in T cell differentiation.

Modification of gap junctions in cells transformed by a temperature-sensitive mutant of Rous sarcoma virus

Prompted by our observation that a reduction in junctional permeance is one of the earlier events in the process of neoplastic transformation of a cell line by Rous sarcoma virus, we analyzed the gap junctions from these cells to determine if the basis of the reduction is a loss of junctional channels. The cells (normal rat kidney, or NRK) are infected with a temperature-sensitive mutant of Rous sarcoma virus, allowing one easily to manipulate the cells into and out of the transformed state, and hence also to manipulate the junctional permeance. Using freeze-fracture electron microscopy, we found that the number and size of the junctions did not change in parallel with the permeance changes we had previously characterized. There is, however, a significant rearrangement of the junctional particles to a more random configuration when the cells are transformed and a reversal to the more ordered pattern when the cells are shifted back to the normal phenotype. These changes do parallel the changes in junctional permeance. We conclude that the permeance of existing junctional channels is modified and that the change in permeance may involve a change in the interaction of the junctional channels with each other and/or the surrounding lipid domain.

Expression of antigen-specific, major histocompatibility complex-restricted receptors by cortical and medullary thymocytes in situ

We have examined the distribution of the antigen-specific, major histocompatibility complex-restricted receptor on mouse thymocytes in situ, using immunohistochemical techniques and the monoclonal antibody KJ16-133. This antibody reacts with the beta chain of the receptors on about 20% of peripheral murine T cells. Of the cortical thymocytes reacting with KJ16-133, cells with only cytoplasmic staining were most frequently observed. Such cytoplasmic staining was not observed in the medulla. Occasional cortical cells had low levels of surface expression, which was almost invariably patched in the region of contact with epithelial cell processes. KJ16-133+ medullary thymocytes had high levels of uniform surface labeling. These results suggest that thymic selection of MHC restriction and/or tolerance may occur in the cortex, where the receptors on maturing thymocytes interact with MHC proteins on epithelial cells.

In situ ultrastructural demonstration of cells bearing Ia antigens in the murine pancreas

Although an important role has been postulated for class II major histocompatibility gene products in the rejection of transplanted islets, the identity of the cells bearing these molecules within pancreatic tissue has been the topic of some controversy. To clearly define the cell population(s) within the murine pancreas that express Ia antigens, perfused murine pancreata were processed for the ultrastructural in situ demonstration of Ia antigens. It was found that Ia antigen expression was restricted to mononuclear cells adjacent to capillaries throughout the exocrine and endocrine portions of the pancreas. Vascular endothelium and ductular epithelium did not express detectable amounts of Ia antigens.

Human cellular src gene: nucleotide sequence and derived amino acid sequence of the region coding for the carboxy-terminal two-thirds of pp60c-src

The nucleotide sequence of the 3' two-thirds of a highly conserved, molecularly cloned human cellular src gene (c-src) has been determined. This region of the c-src gene encodes the tyrosine kinase domain of the cellular src protein (pp60c-src) and corresponds to exons 6 through 12 of the chicken c-src gene, as well as nucleotides 545 to 1542 of the Rous sarcoma virus src gene (v-src). The human c-src sequence is very strongly conserved with respect to both the chicken c-src and the Rous sarcoma virus v-src genes, with nearly 90% nucleotide homology observed in this region. Amino acid sequence conservation in this region is even greater; 98% of the amino acids are conserved between human and chicken c-src. Furthermore, the exon sizes and the locations of the exon-intron boundaries are identical in the human and chicken c-src genes. However, sequences within the introns have not been conserved, and the introns within the human c-src gene are significantly larger than the corresponding introns within the chicken c-src gene. The strong amino acid conservation between the carboxy-terminal two-thirds of pp60c-src of species as divergent as humans and chickens suggests that this portion of the pp60c-src protein specifies one or more functional domains that are of great importance to some aspect of normal cellular growth or differentiation.

Epithelial heterogeneity in the murine thymus: fucose-specific lectins bind medullary epithelial cells

By means of histochemical techniques, two lectins with nominal specificity for L-fucose, Tetragonolobus purpureas agglutinin (TPA), and Ulex europeus agglutinin (UEA) were found to specifically label the medullary area of murine thymuses. Although the binding of both lectins was restricted to the medullary area of the thymus, each staining pattern was unique. Cells binding UEA formed a reticular network throughout the medulla, whereas cells binding TPA occurred as single cells or small clumps of cells and resembled Hassall's corpuscles. The cells binding either lectin were identified as epithelial on the basis of ultrastructural features (tonofilaments, desmosomes, and keratohyalin bodies) and resembled Ia+ medullary epithelial cells described previously. An age-related decline in UEA binding was observed, whereas labeling with TPA remained unchanged. On the basis of the labeling patterns obtained with UEA and TPA and the reported specificities of these two lectins, it is suggested that the majority of the fucose detected is associated with type 1 carbohydrate chains.

Epithelial heterogeneity in the murine thymus: fucose-specific lectins bind medullary epithelial cells

By means of histochemical techniques, two lectins with nominal specificity for L-fucose, Tetragonolobus purpureas agglutinin (TPA), and Ulex europeus agglutinin (UEA) were found to specifically label the medullary area of murine thymuses. Although the binding of both lectins was restricted to the medullary area of the thymus, each staining pattern was unique. Cells binding UEA formed a reticular network throughout the medulla, whereas cells binding TPA occurred as single cells or small clumps of cells and resembled Hassall's corpuscles. The cells binding either lectin were identified as epithelial on the basis of ultrastructural features (tonofilaments, desmosomes, and keratohyalin bodies) and resembled Ia+ medullary epithelial cells described previously. An age-related decline in UEA binding was observed, whereas labeling with TPA remained unchanged. On the basis of the labeling patterns obtained with UEA and TPA and the reported specificities of these two lectins, it is suggested that the majority of the fucose detected is associated with type 1 carbohydrate chains.

The nucleotide sequence of the B gene of bacteriophage Mu

Bacteriophage Mu is a highly efficient transposon which requires the products of the Mu A and B genes in order to transpose at a normal frequency. We have determined the nucleotide sequence of the B gene as well as that of the A-B intergenic region upstream of B. The protein product of the gene contains 312 amino acids and has a predicted molecular weight of 35,061. As expected, there do not appear to be any potential promoter sequences in the intergenic region prior to the gene, but it is preceded by a strong Shine-Dalgarno sequence. The intergenic region does not contain any obvious transcription termination sequences. The frequency of optimal codon usage is similar to that for other transposon and phage genes, and the amino acid composition is comparable to that of an "average" E. coli protein. A region near the amino terminus of the protein resembles the highly conserved bihelical fold which is involved in DNA contact and sequence specific recognition in a number of DNA binding proteins.