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Five hybrids (LSB) were formed between LS174T, a human CEA-producing colonic tumor cell line, and BU25.CAPR, a HeLa derivative which does not produce CEA. All five hybrids produce CEA, but less per cell than LS174T. Approximately 10 % of the chromosomes have been lost from these hybrids. In an attempt to map the gene(s) coding for the protein moiety of CEA, 7 LSPG and 28 LSR hybrids were formed between LS174T and PG19, a mouse melanoma cell line, and LS174T and RAG, a mouse kidney adenocarcinoma cell line, respectively. These hybrids retain between 4 and 21 human chromosomes, and each human chromosome is represented in at least seven hybrids. Two hybrids appeared to produce trace amounts of CEA. These results might represent repression by the mouse genome of CEA production or the production of a structurally abnormal CEA molecule.

It is demonstrated that temperature inactivation of histone synthesis is coupled to inhibition of DNA replication in ts AlS9 and ts Cl mouse L-cells, which are temperature-sensitive (ts) in an S-phase function. In contrast, uncoupling of histone and DNA synthesis occurs in BalB/C-3T3 ts 2 cells which are ts in a function of the pre-DNA-synthetic phase. Termination of histone synthesis in ts AlS9 and ts Cl cells is 16–18 h after onset of temperature inactivation of DNA replication and appears to be associated with general cessation of chromatin replication triggered by the earlier event. Synthesis of histone and other chromosomal proteins proceeds in ts 2 cells under conditions in which DNA synthesis undergoes temperature inactivation. It is suggested that the terminal phenotype of coupled temperature inactivation of DNA and histone synthesis may be diagnostic of cells ts in an S-phase function and may therefore be a useful secondary screen in designation of cell cycle mutants.

Somatic cell hybrids between thymidine kinase-deficient mouse cells and human fibroblasts carrying a translocation of the distal third of the long arm of chromosome 10 to chromosome 17 were studied for the expression of cytoplasmic glutamic-oxaloacetic transaminase. A positive correlation between the expression of human cytoplasmic glutamic-oxaloacetic transaminase and the presence of the distal third of the long arm of chromosome 10 was established.

A series of mouse myeloma cell lines producing mutant γ2b immunoglobin heavy chains, which resemble heavy chain disease proteins, were analyzed for messenger RNA abundance as a function of mRNA alterations. A mutation effectively deleting the γ2b-CH1 domain of the mRNA had little or no effect on Ig heavy chain mRNA abundance on half-life (mutant 10.1). A mutation in the γ2b-CH2 and CH3 domain, causing premature termination of translation, had more deleterious effects on Ig heavy chain mRNA abundance and half-life (mutant 117). Substitution of the deleted portions of the γ2b mRNA with γ2a sequences by subclass switching in the cells (mutants K23 and K25) resulted in increased heavy chain abundance and half-life relative to the parent 117. In contrast, kappa light chain mRNA levels and half-lives remain constant among the mutants. The wild-type and mutant cell lines transcribed the Ig heavy chain γ2b locus equally when compared with an internal β-actin standard by transcription run on studies. Therefore, half-life of the Ig heavy chain mRNA seems to be the principal determinant in cytoplasmic mRNA abundance in this system.

The binding of human interferons to their binding site(s) was measured by the amount of radioiodinated human beta Interferon (HuIFNβ) displaceable by unlabeled human beta, alpha, and gamma Interferon (HuIFNβ, α, and γ). By this approach, HuIFNβ and HuIFNα were found to interact with specific binding sites in cell membranes derived from human cells and mouse-human cell hybrids containing chromosome 21 as their only human chromosome. Specific binding was not observed with cell membranes derived from parental mouse cells or from mouse-human cell hybrids in subsequent generations that have lost human chromosome 21. Although the chromosome 21-positive mouse-human cell hybrids are sensitive to the antiviral effects of HuIFNβ and HuIFNα, they are found to be insensitive to the antiviral effect of HuIFNγ and to lack specific HuIFNγ binding sites. These results suggest that the HuIFNβ and HuIFNα but not HuIFNγ binding sites are coded for by genes located on chromosome 21. The lack of a chromosome 21 gene dosage effect on the inducibility of the antiviral state by HuIFNγ is consistent with this hypothesis.

A cell line, CY-1, was selected in tyrosine free (tyr−) medium after fusion of mouse erythroleukemia (MEL) cells with mitomycin C-treated rat hepatoma cells. MEL cells do not express the enzyme phenylalanine hydroxylase (PH) and are unable to grow in tyr− medium, whereas the rat hepatoma cells constitutively express PH and are able to grow in tyr− medium. CY-1 cells resemble MEL cells morphologically, karyotypically, and in being inducible for hemoglobin synthesis. In contrast to MEL cells, CY-1 expresses PH and is therefore able to grow in tyr− medium. Using a rat cDNA probe for thePH gene, Southern blot analyses were carried out on DNA isolated from CY-1 and parental cells. CY-1 showed the characteristic mousePH gene pattern but the gene copy number was amplified four− to eightfold compared to parental MEL cells.

Giemsa-11 or G-11 is a specialized staining technique utilized to (1) differentiate heterochromatic regions of human chromosomes, (2) identify the presence of human chromosomes in human-rodent hybrid cells, and (3) identify human-rodent translocation products in hybrid cells. Earlier procedures, though useful, are problematic and may fail to yield results due to inadequate differentiation between light and dark staining regions. The improved protocol presented here is easy, reliable, and applicable in both clinical and research situations. A discussion of the biology of the staining process is also given.

The gene for lipoamide dehydrogenase (LD) has been assigned to human chromosome 7 based on filter hybridization analysis of genomic DNA from rodent-human somatic cell hybrids using a cDNA probe for human LD. No indication of multiple copies of the gene was found, in accordance with previous evidence that LD in the pyruvate, α-ketoglutarate, and branched chain α-ketoacid dehydrogenase complexes is genetically as well as biochemically identical.

Galactokinase activity is reduced in 12 independent clones of Chinese hamster ovary cells resistant to 2-deoxygalactose. The frequency of resistant colonies is increased with chemical mutagens. The resistant phenotype is stable in the absence of selection. There is an inverse correlation between the levels of galactokinase activity and the cloning efficiency in deoxygalactose. Cells with high resistance have 1%or less of the enzyme activity observed in the parental cells; while cells with low resistance have 10–30% galactokinase activity. Studies with tetraploid hybrid cells reveal that resistance to deoxygalactose is a recessive trait and that cells with high resistance do not complement those with low resistance. In cell lines with low resistance, the K m for galactose, K i for deoxygalactose, K m for ATP, and thermolability were not significantly altered compared to sensitive parental cells. Although the possibility of mutation at the structural gene locus has not been ruled out, the reduced enzyme activity may also be due to mutation at a regulatory site which affects the number of galactokinase molecules per cell.