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Abstract--Somatic cell hybrids obtained from the fusion of human

We have developed a simple and efficient procedure for transferring specific human genes into mutant Chinese hamster ovary cell recipients that does not rely on using calcium phosphate-precipitated high-molecular-weight DNA. Interspecific cell hybrids between human leukocytes and temperature-sensitive Chinese hamster cell mutants with either a thermolabile leucyl-tRNA synthetase or a thermolabile asparaginyl-tRNA synthetase were used as the starting material in these experiments. These hybrids contain only one or a few human chromosomes and require expression of the appropriate human aminoacyl-tRNA synthetase gene to grow at 39°C. Hybrids were exposed to very high doses of -y-irradiation to extensively fragment the chromosomes and re-fused immediately to the original temperature-sensitive Chinese hamster mutant, and secondary hybrids were isolated at 39°C. Secondary hybrids, which had retained small fragments of the human genome containing the selected gene, were subjected to another round of irradiation, refusion, and selection at 39°C to reduce the amount of human DNA even further. Using this procedure, we have constructed Chinese hamster cell lines that express the human genes encoding either asparaginyl-or leucyl-tRNA synthetase, yet less than 0.1% of their DNA is derived from the human genome, as quantitated by a sensitive dot-blot nucleic acid hybridization procedure. Analysis of these cell lines with Southern blots confirmed the presence of a small number of restriction endonuclease fragments containing human DNA specifically. These cell lines represent a convenient and simple means to clone the human genomic sequences of interest.The genes encoding various components of the protein synthetic machinery in mammalian cells represent a very large family offunctionally related genes. To begin understanding how the expression of this family of genes is coordinated, we are using a combined biochemical and genetic approach to analyze the complex process of protein synthesis. The large number of different Chinese hamster cell mutants that have been isolated with alterations in various protein synthesis components, including ribosomal proteins and aminoacyl-tRNA synthetases, makes this large family of genes especially suitable for genetic analysis and genetic manipulation (1,3,5,13,28,33,34). In Chinese hamster ovary (CHO) cells, two of these genes, leuS and asnS, which encode leucyl-tRNA synthetase and asparaginyl-tRNA synthetase, respectively, are particularly amenable to detailed genetic studies. Conditionally lethal, temperature-sensitive mutants with alterations in either of these genes can be isolated at very high frequencies (1,29,31). In addition, there is a simple counterselective system, growth at an elevated temperature, to isolate revertants in which the temperaturesensitive phenotype of the mutants has been suppressed by second-site intergenic or intragenic mutations. Because of the availability of large numbers of mutants and revertants, the leuS and asnS genes are most amenable to detailed studies o...

As an initial step in determining whether activin could play a role in the development of gonadotroph adenomas, we attempted to determine if activin/inhibin subunit messenger ribonucleic acids (mRNAs) are expressed by these pituitary tumors. We selected 19 pituitary adenomas that had been excised by transsphenoidal surgery and determined by immunocytochemical criteria to be gonadotroph adenomas. Total RNA was extracted from these adenomas and reverse transcribed. The resulting pit-1 complementary DNA, expected to be present only in somatotroph, lactotroph, and thyrotroph cells, was amplified by the polymerase chain reaction to test the possibility that the adenoma tissue was contaminated by normal pituitary tissue. Only the 10 adenomas that did not express pit-1 mRNA were subjected to further analysis by polymerase chain reaction amplification of the adenoma reverse transcriptase products for the activin/inhibin beta B-, beta A-. and alpha-subunits. All 10 adenomas expressed beta B-subunit, none of the 10 expressed the beta A-subunit, and 6 of the 10 expressed the alpha-subunit. We conclude that gonadotroph adenomas express mRNAs for activin/inhibin beta B- and alpha-subunits. The relationship of beta B expression to the pathogenesis of gonadotroph adenomas remains to be determined.

Temperature-resistant revertants, derived from the temperature-sensitive CHO asparaginyl-tRNA synthetase mutant, Asn-5, were isolated and characterized. Several lines of evidence indicate that the temperatureresistant phenotype of the revertants is due to their overproducing the same altered enzyme present in the Asn-5 parent.Conditionally lethal, temperature-sensitive mutants with alterations in the gene encoding asparaginyl-tRNA synthetase (asnRS) can be isolated from two different Chinese hamster cell lines, Chinese hamster ovary (CHO) and V-79 Chinese hamster lung (CHL) cells, at very high frequencies (8, 12). In addition, there is a simple counterselective system, growth at the nonpermissive temperature, to isolate revertants in which the temperature-sensitive phenotype of mutants has been suppressed. The ease with which the gene, asnS, encoding this enzyme can be manipulated makes it one of the most amenable to detailed studies on mutation and reversion at the DNA level. For this reason, as well as a general interest in genes encoding protein synthesis components, we have begun attempts to clone the genomic sequences encoding asnRS.We recently reported the isolation of cell lines derived from a CHO asnRS mutant into which we introduced the normal human asnS gene and in which less than 0.02% of the DNA is derived from the human genome (5). We have since prepared a complete lambda genomic DNA library from this cell line and have isolated a group of 160 recombinant phage containing human DNA, which should represent all of the human sequences present in this cell line, including those encoding asnRS. To aid in screening this group of recombinants, we were interested in isolating Chinese hamster cell lines with elevated levels of asnRS, and hopefully elevated levels of asnRS mRNA, from which we could prepare a cDNA library enriched for these sequences.Isolation of temperature-resistant revertants from a temperature-sensitive asnRS mutant. The lack of availability of specific inhibitors of asnRS precluded the selection of cell lines that overproduced this enzyme based upon increased resistance to substrate analogs (1, 3, 11). As an alternate approach to obtaining such cell lines, we isolated temperature-resistant revertants from the CHO asnRS mutant 8), which is nonviable at 39°C in medium lacking asparagine but has a normal generation time at 34°C. It seemed possible that variants of Asn-5 which greatly overproduced the defective enzyme were viable at 39°C and could be recovered as temperature-resistant revertants. To increase the likelihood of isolating cell lines which overproduced asnRS as a result of their having amplified the asnS gene, we pretreated cultures of Asn-5 cells with the phorbol ester 12-