Cloning of a functional human adenine phosphoribosyltransferase (APRT) gene: identification of a restriction fragment length polymorphism and preliminary analysis of DNAs from APRT-deficient families and cell mutants

A complete human APRT gene has been isolated from a lambda phage genomic library using cloned mouse APRT DNA as a probe. The human gene, contained in a recombinant lambda phage designated lambda Huap15, is functional by virtue of its capacity to transfer human APRT activity to Aprt- mouse recipient cells after phage-mediated transfection. Digestion of lambda Huap15 DNA with BamH1 generated a 2.2-kb fragment that is the only fragment of eight produced to hybridize with the mouse APRT gene. This 2.2-kb BamH1 fragment is a unique, single copy sequence, and has been used to identify a restriction fragment length polymorphism (RFLP) associated with the APRT locus. Taq1 digestion and Southern blot analysis of DNAs from 49 unrelated individuals produced three different patterns. DNAs of 30 individuals produced a restriction pattern of three labeled fragments about 500 bp, 600 bp, and 2.1 kb in size, which is characteristic for individuals homozygous for the more common allele. Two individuals homozygous for the less frequent allele displayed labeled fragments of 500 bp and 2.7 kb. The remaining 17 DNA samples produced all four labeled bands as expected for heterozygous individuals. The frequency of heterozygotes in the population is about 35%, while the frequency of the less common allele is about 0.21. Restriction enzyme analysis of DNAs from two APRT-deficient brothers and from an unrelated heterozygote revealed no gross deletions or rearrangements, nor the Taq1 polymorphism.

Plasmid, phage, and genomic DNA-mediated transfer and expression of prokaryotic and eukaryotic genes in cultured human cells

Transfection of mammalian cells with genomic DNA and cloned genes is now relatively routine. However, the vast majority of studies have used rodent cells as recipients. Here we describe efficient transfection of two human cell lines, the hypoxanthine guanine phosphoribosyltransferase (HPRT)-deficient HeLa line, D98/AH-2, and the adenine phosphoribosyltransferase (APRT)-deficient HT1080 line, HTD114. D98/AH-2 cells were transfected with the pSV2-gpt plasmid of Mulligan and Berg, which contains the E. coli xanthine-guanine phosphoribosyltransferase (gpt) gene, and Gpt + transfectants were selected in HAT medium. HTD114 cells were transfected with (1) genomic hamster DNA, and ouabain resistant transfectants were selected in 5 X 10(-7)M ouabain; (2) with hamster and mouse genomic DNA, and Aprt + cells were selected in AAA medium; (3) with plasmids containing either the cloned hamster or mouse APRT genes, and Aprt + cells were selected; and (4) with phage particles containing a cloned mouse APRT gene, and Aprt + cells were selected. Transfection efficiencies ranged from 0.25 to 1.5 X 10(3) transfectants per microgram DNA, and in certain cases secondary transfections were done. Foreign DNA in recipients was detected by blot hybridization, and the expression of foreign genes was detected by cell growth in selective media and the expression of enzymes characteristic of the species of the donor DNA. The majority of transfectants showed stable expression of the transgenome.

Cloning and expression of a mouse adenine phosphoribosyltransferase gene

A functional mouse adenine phosphoribosyltransferase (APRT) gene was identified and cloned by screening a mouse sperm genomic DNA library in lambda Charon 4A. The probe utilized for screening was a restriction fragment encoding much of the hamster APRT gene. Six recombinants that hybridized with the probe were identified, and after digestion with restriction enzymes EcoRI and PvuII revealed three different patterns of digestion for each enzyme. Of the six recombinants, five representing two of the restriction patterns possessed transforming activity. A sixth recombinant, which has a unique restriction pattern, lacks transforming activity but hybridizes well with hamster APRT coding sequences and is a possible candidate for a pseudogene. We used three criteria for conclusively identifying the mouse APRT genes. (1) DNA from the recombinant lambda phage hybridizes with DNA encoding hamster APRT. (2) The recombinant lambda phages and their DNAs transform mouse, hamster and human APRT- cells to the APRT+ phenotype. (3) The hamster and human transformants display APRT activity that migrates with a mobility characteristic of mouse APRT and not of hamster or human. A 3.1-kb EcoRI-SphI restriction fragment which retains transforming activity has been subcloned into the plasmid pBR328. Comparison of restriction enzyme sites with those contained in a mouse APRT cDNA, coupled with loss of transforming activity after enzyme digestion, indicates that the mouse APRT gene is larger than 1.8 kb and contains at least three introns.

Hypersensitivity of DNA in transcriptionally active chromatin to ionizing radiation

We have examined the size distribution of single-strand fragments of total 3H-labeled DNA and of DNA sequences complementary to specific probes in gamma-irradiated and unirradiated mouse L929 cells. Those DNA sequences which hybridize to rDNA or to poly(A+)RNA have lower number average molecular weights and sustain 5--6-times the number of single-strand breaks as do satellite DNA sequences or the bulk DNA. We therefore conclude that transcriptionally active DNA sequences are more susceptible to ionizing radiation-induced damage than are inactive sequences, and suggest that these differential susceptibilities are a likely consequence of differences in their chromatin organization.

Interspersion of mouse satellite deoxyribonucleic acid sequences

DNA sequences with homology to the major (A + T)-rich mouse satellite component were localized in CsCl gradients by hybridization with a labeled satellite cRNA probe. Although, as expected, most of the hybridization was to DNA in the satellite-rich shoulder, substantial radioactive cRNA hybridized with DNA from denser regions of the gradient. Further examination revealed that hybridization to main-band DNA was not due to physical trapping of satellite DNA in the gradient, and melting experiments argue that the associated radioactivity was due to true RNA/DNA hybridization. Nearest-neighbor analysis of hybridized [alpha-32P]CTP-labeled l-strand cRNA indicates that hybridization to main-band DNA is by the satellite cRNA and not a contaminant. Together, these data argue that mouse satellite-like sequences are interspersed within the main-band fraction of DNA. For the support of this contention, total mouse DNA, purified main-band DNA, and purified satellite DNA were digested with EcoRI, sedimented in a sucrose gradient, and hybridized with labeled satellite cRNA. Mouse satellite DNA is not cleaved with EcoRI, so that purified EcoRI-digested satellite DNA sediments as a high molecular weight component. When total mouse DNA is digested with EcoRI, the majority of satellite-like sequences remain as high molecular weight DNA; however, significant amounts of satellite-like sequences sediment with the bulk of the lower molecular weight digested DNA, lending further credence to the argument that satellite-like sequences are interspersed with main-band DNA.

Replication of mammalian DNA in bromodeoxyuridine: appearance of a component with intermediate density

When three lines of mammalian cells were cultured with 5-bromodeoxyuridine (BrdUrd) for less than one generation, their DNAs displayed three peaks in CsCl gradients. In addition to the expected unsubstituted (LL) and hybrid (LH) peaks, there was a significant absorbance peak of intermediate density (INT) between LH and LL DNAs. This INT DNA has characteristics expected of an intermediate of DNA replication. Upon shearing, it behaves as though it contains contiguous segments of unsubstituted and hybrid DNAs. Upon continuous exposure of cells to [3H]-BrdUrd, radioactivity accumulates in INT DNA for 60-90 min when a steady state condition is reached. At that time, the rate of incorporation into LH DNA increases, consistent with a precursor-product relationship. In a pulse-chase experiment, radioactivity is chased from INT DNA into LH DNA. To account for the above observations and for the size and sharpness of the INT DNA peak in CsCl, we suggest that a high molecular weight replication intermediate accumulates before completing replication into mature daughter molecules.

Enrichment and visualization of small replication units from cultured mammalian cells

DNA from cultured Chinese hamster cells has been fractionated to yield a population of DNA enriched for replicating molecules. Molecules containing replication structures were analyzed by electron microscopy, and replicon size was estimated. The enrichment procedure takes advantage of single-stranded regions characteristic of replicating molecules, and the greater affinity of mercuric ion for single-stranded rather than native DNA. After interaction with low concentrations of HgCl2, DNA with bound mercury is separated from the bulk of the DNA by virtue of its increased buoyant density in an isopycnic Cs2SO4 gradient. When DNA from cells labeled with [3H]thymidine for 45 s is interacted with HgCl2 and banded in Cs2SO4, the DNA with the highest specific activity is found in a dense region of the gradient. The high specific activity DNA behaves kinetically like nascent DNA since the radioactivity can be chased into main band if the cells are incubated for a further 2 h in excess unlabeled thymidine. Electron microscope analysis of the DNA in the enriched fraction confirmed that it contains a substantial fraction of molecules with replication structures. The level of enrichment is about 25-fold compared to unfractionated DNA or DNA taken from the main band of the Hg++/Cs2SO4 gradient. Of the replicating molecules visualized, 85% possessed a single replication structure. All molecules with multiple replication forms contained replicon sizes less than 5 micron, ranging from 0.2 to 4.5 micron. Replicon size was determined by measuring the distance from the center of one replication structure to the center of the adjacent replication structure on the same molecule. The replicons observed in this study are far smaller than can be detected by DNA fiber autoradiography and are in the same size range as the very small replication units reported in embryonic systems.

Heterogeneity in Chinese hamster ribosomal DNA

A discrete heterogeneity has been detected in Chinese hamster ribosomal DNA after Eco R1 digestion of total DNA followed by a Southern transfer and hybridization with [125I]18S or [125I]28S ribosomal RNA. Digestion with Eco R1 produces three fragments, 4.3, 6.0 and 9.5 x 10(6) daltons respectively, which hybridize with 18S RNA. The smallest fragment also hybridizes with 28S RNA. Either length heterogeneity or sequence heterogeneity (i.e. presence of an additional Eco R1 site in some of the rDNA molecules) must be invoked to account for the two larger Eco R1 fragments that contain 18S but not 28S sequences. Eco R1 and Hind III maps, consistent with either length or sequence heterogeneity, are presented. The data at this time, however, do not distinguish between the two alternatives.

Organisation of the genes coding for 5S RNA in the Chinese hamster

Hybridisation of iodinated mouse 5S RNA with Chinese hamster DNA fractionated on a CsCl gradient, identifies two separate DNA components that hybridise with the labelled probe. One component is slightly, and one considerably heavier than the main band DNA. This suggests that the gene coding for 5S RNA in the Chinese hamster may constitute two distinct populations differing in G + C content of their adjacent spacer regions.

The effect of potassium on the cell membrane potential and the passage of synchronized cells through the cell cycle

The cell membrane potential of cultured Chinese hamster cells is known to increase at the start of the S phase. The putative role of the cell membrane potential as a regulator of cell proliferation was examined by following the cell cycle traverse of synchronized Chinese hamster cells in the presence or absence of high exogenous levels of potassium. An increase in external potassium levels results in a depressed membrane potential and a reduced rate of cell proliferation. A potassium concentration of 115 mM was used in experiments with synchronized cells since at that level cell proliferation is almost completely halted, recovery of growth is rapid and complete, and the membrane potential is reduced to a level well below that normally found in cells in the G1 phase. A mitotic population was divided into four aliquots and plated in either control medium or medium containing 115 mM K+. Cells placed directly into high K+ medium were retarded in their exit from mitosis and displayed a delayed and abnormal entry into the S phase. If control medium was added after two hours, cell cycle traverse was normal, but delayed by two hours compared to control cells. If the mitotic cells were plated directly into control medium and two hours later were shifted to high K+ medium, the cells entered the S phase in the absence of the normally observed increase in membrane potential and proceeded to the next mitosis normally. It was concluded that the increase in membrane potential observed at the start of the S phase in isolated synchronized cells is not a requirement for the initiation of DNA synthesis. In addition, sensitivity to the high potassium regimen was found at two different times during the cell cycle. In one case, cells were impeded in their transit through mitosis. Such cells displayed an altered chromosome structure which may account for the partial mitotic block. In the second case, synchronized cells displayed a sensitivity to the high potassium regimen in early G1 which appeared to be separate from the block in mitosis and independent of a change in the membrane potential.

Cell cycle specific fluctuations in adenosine 3':5'-cyclic monophosphate and polyamines of Chinese hamster cells

Chinese hamster V79 cells were synchronized by mitotic selection, which resulted in approximately 95% synchrony. The adenosine 3':5'-cyclic monophosphate level was elevated within 3 hr (G1 phase) and reached a level 2-fold higher than in early G1 within 6 hr (early S phase). An increase in ornithine decarboxylase activity (6-ornithine carboxy-lyase, EC 4.1.1.17), the initial enzyme in the polyamine biosynthetic pathway, was detected within 4 hr and was maximal at 8 hr. Since about 20% of the cells were labeled with [3-H]thymidine at 4 hr, ornithine decarboxylase exhibits cell-cycle specific activity starting in late G1 and continuing through middle S phase. The activity of S-adenosylmethionine decarboxylase (S-adenosyl-L-methionine carboxylase, EC 4.1.1.50) increased within 5 hr, i.e., early S phase. It is suggested on the basis of these data and other studies discussed herein that the increase in ornithine decarboxylase activity, which parallels closely the elevation in cyclic AMP, is an example of adenosine 3':5'-cyclic monophosphate-mediated protein synthesis.

Induced changes in the rates of uridine- 3 H uptake and incorporation during the G 1 and S periods of synchronized Chinese hamster cells

The rates of uridine-5-(3)H incorporation into RNA and the rates of uridine uptake into the acid-soluble pool during the cell cycle of V79 Chinese hamster cells were examined. Cells cultured on Eagle's minimal essential medium supplemented with fetal calf serum, lactalbumin hydrolysate, glutamine, and trypsin displayed rates of incorporation and uptake which increased only slightly during G(1) and accelerated sharply as DNA synthesis commenced. In contrast, cells cultured on minimal essential medium supplemented only with calf serum exhibited rates of incorporation and uptake which increased linearly through both G(1) and S. The transition from one pattern to the other can be induced within 24 hr and is completely reversible. The nonlinear pattern exhibited by cells grown on the supplemented fetal calf serum medium can also be overcome with high exogenous uridine concentrations. In the presence of 200 microM uridine, these cells display a linear pattern of increase in rates of uridine incorporation and uptake. It is concluded that at lower uridine concentrations the pattern of increase in the rate of uridine incorporation into RNA during the cell cycle for a given population of cells is dependent upon the rate of uridine entry into the cell, and that this pattern is not rigidly determined but can be modified by culture conditions.