A unique pattern of integrated viral genes in hamster cells transformed by highly oncogenic human adenovirus 12

Human adenovirus type 9-induced rat mammary tumors

Following subcutaneous inoculation of newborn Wistar-Furth rats with human adenovirus type 9 (Ad9), 16 of 16 female and 0 of 11 male rats developed mammary tumors. Tumor-positive animals usually developed tumors in multiple glands. Histopathological analyses indicated that three general categories of tumor could be identified. Mammary fibroadenomas were the most common tumor type encountered, but phyllodeslike tumors and solid sarcomas were also frequently found. In situ hybridization and immunohistochemical techniques established that benign fibroadenomas were derived from mammary fibroblasts (collagen type I- and vimentin-positive cells) and that malignant tumors were derived from myoepithelial cells (collagen type IV-, vimentin-, and muscle-specific actin-positive cells). The fact that mammary tumors were limited to female rats suggested that female hormones are essential for tumor growth and development. In this regard, ovariectomy of Ad9-infected female rats prevented tumor development, while subsequent diethylstilbestrol (DES) treatment elicited tumor formation. In addition, Ad9-infected and castrated male rats which received DES also developed mammary tumors. Established male mammary tumors regressed when DES treatment was stopped and reappeared after DES treatment was resumed. Together, these results indicate that estrogen is required for both initiation and maintenance of Ad9-induced mammary tumors. Southern blot analysis of high-molecular-weight tumor DNA showed that mammary tumor cells contained single or multiple integrated copies of the entire Ad9 genome. RNase protection experiments established that estrogen receptor as well as Ad9 E1a and E4 mRNAs were expressed in mammary tumors, but Ad9 E3 and, surprisingly, E1b mRNAs were not expressed at detectable levels.

Identification of adenovirus 12-encoded E1A tumor antigens synthesized in infected and transformed mammalian cells and in Escherichia coli

A 16-amino acid peptide, H2N-Arg-Glu-Gln-Thr-Val-Pro-Val-Asp-Leu-Ser-Val-Lys-Arg-Pro-Arg-Cys-COOH (peptide 204), targeted to the common C-terminus of human adenovirus 12 (Ad12) tumor antigens encoded by the E1A 13S mRNA and 12S mRNA, has been synthesized. Antibody prepared in rabbits against peptide 204 immunoprecipitated two proteins of apparent Mr 47,000 and 45,000 from extracts of [35S]methionine-labeled Ad12-early infected KB cells and a 47,000 protein from extracts of the Ad12-transformed hamster cell line, HE C19. Immunoprecipitation analysis of infected and transformed cells labeled with 32Pi showed that both major Ad12 E1A T antigens are phosphoproteins. Immunofluorescence microscopy of Ad12-early infected KB cells with antipeptide antibody showed the site of E1A protein concentration to be predominantly nuclear. E1A proteins were detected by immunofluorescence at 4 to 6 h postinfection and continued to increase until at least 18 h postinfection. Antipeptide 204 antibody was used to analyze the proteins synthesized in Escherichia coli cells transformed by plasmids containing cDNA copies of the Ad12 E1A 13S mRNA or 12S mRNA under the control of the tac promoter (D. Kimelman, L. A. Lucher, M. Green, K. H. Brackmann, J. S. Symington, and M. Ptashne, Proc. Natl. Acad. Sci. U.S.A., in press). A major protein of ca. 47,000 was immunoprecipitated from extracts of each transformed E. coli cell clone. Two-dimensional gel electrophoretic analysis of immunoprecipitates revealed that the T antigens synthesized in infected KB cells, transformed hamster cells, and transformed E. coli cells possess very similar molecular weights and acidic isoelectric points of 5.2 to 5.4.

Isolation and characterization of four adenovirus type 12-transformed human embryo kidney cell lines

Four transformed cell lines were established from cultures of human embryo kidney (HEK) cells microinjected or transfected with cloned adenovirus 12 (Ad12) EcoRI-C DNA (0 through 16.5 map units of the left-hand end of the viral genome). Each cell line showed a different growth pattern. Southern blotting demonstrated that all of the cell lines contained Ad12-specific DNA sequences, but in the microinjected isolates these were at a much lower copy number than in the transfected isolate. Two cell lines (Ad12 HEK 1 and 3) appeared to contain tandemly repeated Ad12 EcoRI-C DNA fragments. Immunoprecipitation and Western blotting confirmed that Ad12 early region 1 (E1) proteins were being expressed by all four of the transformed cell lines, but indicated that E1A polypeptide expression was considerably less than E1B polypeptide expression. All of the Ad12-transformed HEK cell lines were tumorigenic when inoculated intracranially into athymic nude mice.

Isolation and characterization of four adenovirus type 12-transformed human embryo kidney cell lines

Four transformed cell lines were established from cultures of human embryo kidney (HEK) cells microinjected or transfected with cloned adenovirus 12 (Ad12) EcoRI-C DNA (0 through 16.5 map units of the left-hand end of the viral genome). Each cell line showed a different growth pattern. Southern blotting demonstrated that all of the cell lines contained Ad12-specific DNA sequences, but in the microinjected isolates these were at a much lower copy number than in the transfected isolate. Two cell lines (Ad12 HEK 1 and 3) appeared to contain tandemly repeated Ad12 EcoRI-C DNA fragments. Immunoprecipitation and Western blotting confirmed that Ad12 early region 1 (E1) proteins were being expressed by all four of the transformed cell lines, but indicated that E1A polypeptide expression was considerably less than E1B polypeptide expression. All of the Ad12-transformed HEK cell lines were tumorigenic when inoculated intracranially into athymic nude mice.

Transformation by human adenoviruses

When, approximately 10 years ago, it was shown that the functions essential for cell transformation were localized in a small region of the adenovirus genome, a DNA segment which at that time was thought to be capable of encoding two or three average-sized proteins at most, it seemed reasonable to hope that an understanding of the mechanisms by which adenoviruses transform cells might be quickly achieved. While such optimism might be forgiven, it was quite clearly naive in the extreme. As a consequence of mRNA splicing and the use of overlapping reading frames the number of proteins encoded within E1 is 2-3-times greater than would have been predicted a decade ago, and post-translational modifications may add another dimension of complexity. In fact it has taken nearly all of the past decade just to identify the proteins encoded in E1 and to characterize them in the most rudimentary way. However, we have now entered a period in which new information is accumulating at an extremely rapid rate as a result of several major technical and fundamental advances. Chief among these are the use of recombinant DNA techniques, particularly site-directed mutagenesis, which combined with methods for introducing mutations made in cloned sequences back into infectious virus, clearly represents a powerful approach to studying the functions of transforming proteins. In addition, the ability to express transforming proteins in bacteria and to produce large amounts of highly purified proteins which previously were only just detectable in infected and transformed cells is a major breakthrough. Advances in immunological techniques, particularly the development of monoclonal antibodies and antisera against synthetic peptides, have enormously simplified the task of detecting and characterizing E1 proteins. Finally, recent results suggesting that adenovirus transforming proteins may be functionally and structurally similar to other oncogenes brings a new perspective to the study of oncogenic transformation. Have all the proteins involved in transformation by adenoviruses been identified? It seems probable that all those virally coded proteins which play a major role are now known but of course minor players in the cast could still be waiting in the wings. We have pointed out that viral functions encoded outside region E1 may have some importance at least in initiation of transformation by virions and have speculated on the possibility that one or more of these may be involved in the integration of viral DNA into the host cell chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenovirus type 12-transformed rat embryo brain and rat liver epithelial cell lines: adenovirus type 12 genome content and viral protein expression

By Southern blotting analysis, six adenovirus type 12 (Ad-12)-transformed rat embryo brain cell lines and one Ad-12-transformed rat liver epithelial line were shown to contain all or nearly all the Ad-12 genome. Another Ad-12 rat liver epithelial cell line contained a repeating structure consisting of only the left-hand 16% of the Ad-12 genome. Three Ad-12-specified proteins (molecular weights, 52,000, 41,000, and 18,000) were found by immunoprecipitation to be common to all of these cell lines. Two rat liver epithelial lines, produced from an Ad-12-infected culture and previously shown to be T-antigen negative by immunofluorescence, contained no detectable Ad-12 genome or Ad-12-specified proteins. Although some of the rat embryo brain transformants had been shown previously to express "late" Ad-12 mRNA, no Ad-12 structural proteins were found to be produced by these cell lines.

Integration of adenovirus type 2 DNA at a limited number of cellular sites in productively infected cells

Several experimental findings supported the notion that adenovirus type 2 DNA is covalently linked to cellular DNA in productively infected human cells. Although the significance of this linkage for the process of viral replication is unknown, the recombination of viral DNA with host DNA may simply reflect the efficacy of the recombination systems in mammalian cells. In this report, evidence is presented for the existence of selective sites of recombination between viral and host DNAs. These sites are presumably located in repetitive sequences of cellular DNA. All parts of the viral genome appear to be represented in the recombinant form.

Integration sites of adenovirus type 12 DNA in transformed hamster cells and hamster tumor cells

The patterns and sites of integration of adenovirus type 12 (Ad12) DNA were determined in three lines of Ad12-transformed hamster cells and in two lines of Ad12-induced hamster tumor cells. The results of a detailed analysis can be summarized as follows. (i) All cell lines investigated contained multiple copies (3 to 22 genome equivalents per cell in different lines) of the entire Ad12 genome. In addition, fragments of Ad12 DNA also persisted separately in non-stoichiometric amounts. (ii) All Ad12 DNA copies were integrated into cellular DNA. Free viral DNA molecules did not occur. The terminal regions of Ad12 DNA were linked to cellular DNA. The internal parts of the integrated viral genomes, and perhaps the entire viral genome, remained colinear with virion DNA. (iii) Except for line HA12/7, there were fewer sites of integration than Ad12 DNA molecules persisting. This finding suggested either that viral DNA was integrated at identical sites in repetitive DNA or, more likely, that one or a few viral DNA molecules were amplified upon integration together with the adjacent cellular DNA sequences, leading to a serial arrangement of viral DNA molecules separated by cellular DNA sequences. Likewise, in the Ad12-induced hamster tumor lines (CLAC1 and CLAC3), viral DNA was linked to repetitive cellular sequences. Serial arrangement of Ad12 DNA molecules in these lines was not likely. (iv) In general, true tandem integration with integrated viral DNA molecules directly abutting each other was not found. Instead, the data suggested that the integrated viral DNA molecules were separated by cellular or rearranged viral DNA sequences. (v) The results of hybridization experiments, in which a highly specific probe (143-base pair DNA fragment) derived from the termini of Ad12 DNA was used, were not consistent with models of integration involving true tandem integration of Ad12 DNA or covalent circularization of Ad12 DNA before insertion into the cellular genome. (vi) Evidence was presented that a small segment at the termini of the integrated Ad12 DNA in cell lines HA12/7, T637, and A2497-3 was repeated several times. The exact structures of these repeat units remained to be determined. The occurrence of these units might reflect the mechanism of amplification of viral and cellular sequences in transformed cell lines.

Patterns of viral DNA integration in cells transformed by wild type or DNA-binding protein mutants of adenovirus type 5 and effect of chemical carcinogens on integration

The integration pattern of viral DNA was studied in a number of cell lines transformed by wild-type adenovirus type 5 (Ad5 WT) and two mutants of the DNA-binding protein gene, H5ts125 and H5ts107. The effect of chemical carcinogens on the integration of viral DNA was also investigated. Liquid hybridization (C(0)t) analyses showed that rat embryo cells transformed by Ad5 WT usually contained only the left-hand end of the viral genome, whereas cell lines transformed by H5ts125 or H5ts107 at either the semipermissive (36 degrees C) or nonpermissive (39.5 degrees C) temperature often contained one to five copies of all or most of the entire adenovirus genome. The arrangement of the integrated adenovirus DNA sequences was determined by cleavage of transformed cell DNA with restriction endonucleases XbaI, EcoRI, or HindIII followed by transfer of separated fragments to nitrocellulose paper and hybridization according to the technique of E. M. Southern (J. Mol. Biol. 98: 503-517, 1975). It was found that the adenovirus genome is integrated as a linear sequence covalently linked to host cell DNA; that the viral DNA is integrated into different host DNA sequences in each cell line studied; that in cell lines that contain multiple copies of the Ad5 genome the viral DNA sequences can be integrated in a single set of host cell DNA sequences and not as concatemers; and that chemical carcinogens do not alter the extent or pattern of viral DNA integration.

Adenovirus type 12-induced rat tumor cells of neuroepithelial origin: persistence and expression of the viral genome

Four cell lines derived from adenovirus type 12-induced rat brain tumors were studied. The polyploid cells displayed neuroepithelial characteristics and were transplantable into syngeneic rats and nude mice. In tissue culture the cells grew in monolayers and multilayers. A very high saturation density was reached, and the cells plated in agar and were easily agglutinated with low concentrations of concanavalin A. Between 2 and 11 copies of the viral genome per diploid cellular genome were detected by reassociation kinetics analysis in the different lines. The patterns of distribution of viral DNA sequences in these lines, as revealed by blot analysis, suggest colinear integration of the intact viral genome into the cellular DNA. The patterns of integration were stable after more than 15 months of prolonged tissue culture and after animal reimplantation. Integration patterns were identical in three of the tumor lines and different in another line. Viral sequences were transcribed. The extent of homology found toward adenovirus type 12 DNA in polyadenylated polysome-associated mRNA isolated from the tumor lines suggests that the early and some of the late genes of adenovirus type 12 DNA are transcribed in these tumor cells. Infectious virus was not rescuable from these lines.

Viral DNA sequences and gene products in hamster cells transformed by adenovirus type 2

Complementary strand-specific adenovirus DNA of full length or from endonuclease BamHI fragments was used as a probe to estimate the fractional representation and abundance of viral sequences in five hamster cell lines (Ad2HE1-5) transformed with UV-inactivated adenovirus type 2. The fraction of the viral genome present in the five transformed cell lines varied from 44% in the Ad2HE5 cell line to 84% in the Ad2HE3 cell line. The number of viral DNA copies per diploid cell equivalent ranged from 1.8 in the Ad2HE1 line to 7.1 in the Ad2HE4 line. In vivo labeling with [35S]methionine followed by immunoprecipitation with an antiserum against adenovirus type 2 early proteins revealed virus-specific polypeptides with molecular weights of 42,000 to 58,000 in extracts from all five hamster cell lines. Several other early viral polypeptides were detected in some of the adenovirus type 2-transformed hamster cell lines.

Patterns of integration of viral DNA sequences in the genomes of adenovirus type 12-transformed hamster cells

The patterns of integration of the viral genome have been analyzed in four hamster cell lines transformed by adenovirus type 12 (Ad12). It has previously been shown that in each of the cell lines HA12/7, T637, A2497-2 and A2497-3, the viral genome persists in multiple copies, and that different parts of the viral DNA are represented non-stoichiometrically (Fanning and Doerfler, 1976). All four cell lines are oncogenic when injected into hamsters. The DNA from each of the cell lines was extracted and cleaved in different experiments with restriction endonucleases Bam HI, Bgl II, Eco RI, Hind III, Hpa II or Sma I. The DNA fragments were separated on 1% agarose slab gels and transferred to nitrocellulose filters by the Southern technique. Ad12 DNA sequences were detected by hybridization to Ad12 DNA, which was 32 P-labeled by nick translation, and by subsequent autoradiography. In some experiments, the 32P-labeled Eco RI restriction endonuclease fragments of Ad12 DNA were used to investigate the distribution of specific segments of the viral genome in the cellular DNA. For each cell line, a distinct and specific pattern of integrated viral DNA sequences is observed for each of the restriction endonucleases used. Moreover, viral sequences complementary to the isolated Eco RI restriction endonuclease fragments are also distributed in patterns specific for each cell line. There are striking differences in integration patterns among the four different lines; there are also similarities. Because the organization of cellular genes in virus-transformed as compared to normal cells has not yet been determined, conclusions about the existence or absence of specific integration sites for adenovirus DNA appear premature. Analysis of the integration patterns of Ad12 DNA in the four hamster lines investigated reveals that some of the viral DNA molecules are fragmented prior to or during integration. Analysis with specific restriction endonuclease fragments demonstrates that the Eco RI B, D and E fragments, comprising a contiguous segment from 0.17-0.62 fractional length units of the viral DNA, remain intact during integration in a portion of the viral DNA molecules. Although each cell line carries multiple copies of Ad12 DNA, the viral DNA sequences are concentrated in a small number of distinct size classes of fragments. This finding is compatible with, but does not prove, the notion that at least a portion of the viral DNA sequences, is integrated into repetitive sequences, or else that the integrated viral sequences have been amplified after integration. In the three cell lines which were tested, the integration pattern is stable over many generations, with continuous passage-twice weekly-of cells for 6-7 months. In the three cell lines which were examined, the integration pattern is identical in a number of randomly isolated clones. Hence it can be concluded that the patterns of integration are identical among all cells in a population of a given line of transformed cells.

Adenovirus type 12 DNA sequences in primary hamster tumors

In five out of six primary hamster tumors induced by adenovirus type 12, less than 55% of the adenovirus type 12 genome is present. Various fragments of the integrated viral DNA were present in non-equimolar amounts.

Multiple copies of human adenovirus 12 genomes are integrated in virus-induced hamster tumors

Tumors induced in hamsters by highly oncogenic human adenovirus 12 contain multiple copies of 90 to 100% of the viral genome in an integrated form.

Integrated viral sequences in adenovirus type 12-transformed hamster cells

The physical state of the viral genome in four lines of hamster cells transformed by adenovirus type 12 (Ad12) has been investigated. The four lines of transformed cells originated from hamster cells after infection with Ad12 at multiplicities ranging from 5-350 plaque-forming units per cell. The DNA from transformed cells has been restricted with the Sal I endonuclease from Streptomyces albus which cleaves adenovirus DNA more frequently than DNA from adenovirus-transformed hamster cells. Thus after cleavage by the Sal I enzyme, it is possible to separate free adenovirus DNA sequences from these which are covalently linked to cellular DNA in transformed hamster cells. The results of sequential hybridization experiments in which the Sal I-treated DNA from transformed cells is first annealed to Ad12 DNA on filters, then eluted, and finally hybridized to hamster cell DNA, support the model of Ad12 DNA integrated in multiple fragments into the host genome. Further experiments will be required to characterize the host sequences adjacent to adenovirus DNA and to compare these sequences in different lines of Ad12 transformed cells.

Do highly oncogenic group A human adenoviruses cause human cancer? Analysis of human tumors for adenovirus 12 transforming DNA sequences

Adenovirus 12 (Ad12) (Huie) (highly oncogenic group A) readily induces tumors in newborn rodents. Since Ad12 is isolated from human fecal samples, we investigated whether it plays a role in the etiology of human gastrointestinal cancer. If Ad12 is a causal agent of human cancer, then human tumors should contain Ad12 transforming genes, as indicated by studies of cells transformed in vitro and in vivo by oncogenic viruses. Ad12 DNA and the Ad12 transforming restriction fragment (EcoRI-C fragment, left 16% of the viral genome) were labeled in vitro to 10(7) to 4 X 10(8) cpm/mug by the nick translation reaction of DNA polymerase of Escherichia coli. The fidelity and sensitivity of these probes were established by (i) analysis of DNA from Ad12-transformed cells and from hamsters with tumors induced by Ad12, (ii) reconstruction experiments with added Ad12 DNA and EcoRI restriction fragments, and (iii) comparison of annealing characteristics with Ad12 probes labeled in vivo. With Ad12 [3H]DNA as probe, no viral DNA sequences were detected in 18 normal gastrointestinal tissues and 34 gastrointestinal tumors, including cancers of the colon, rectum, small intestine, and stomach, under conditions that would detect 0.1 copy of the Ad12 genome per tumor cell. Similar analyses of Ad12-transformed hamster cells and Ad12 primary hamster tumors indicated 6-18 copies per cell of over 90% of the viral genome. With the Ad12 EcoRI-C transforming fragment as probe, no hybridization was detected with 32 human gastrointestinal tumors and five normal tissues; this result excludes 1-2% of the Ad12 genome per tumor cell. Our date are strong evidence that Ad12 is not a major cause of human gastrointestinal cancer. The Ad12 transforming EcoRI-C fragment hybridized (50-68% efficiency) with other Ad12 isolates and with Ad18 and 31 (members of oncogenic group A), but not at all with 28 other human Ad serotypes (manuscript in preparation). Thus other group A members probably are also not involved in human gastrointestinal cancer. No viral DNA sequences were detected in 12 normal lungs and 22 lung tumors, suggesting that respiratory cancer does not involve an Ad12 etiology.

Transcription of the genome of adenovirus type 12. III. Maps of stable RNA from productively infected human cells and abortively infected and transformed hamster cells

The adenovirus type 12-specific mRNA and the stable nuclear RNA from productively infected KB cells, early postinfection, from abortively infected BHK-21 cells, and from the adenovirus type 12-transformed hamster lines T637 and HA12/7 have been mapped on the genome of adenovirus type 12. The intact separated heavy (H) and light (L) strands of adenovirus type 12 DNA have been used to determine the extent of complementarity of the mRNA or nuclear RNA from different cell lines to each of the strands. More precise map positions have been obtained by the use of the H and L complements of the fragments of adenovirus type 12 DNA which were produced with the EcoRI and BamHI restriction endonucleases. The results of the mapping experiments demonstrate that the mRNA's isolated early from productively and abortively infected and from two lines of transformed cells are derived from the same or similar regions of the adenovirus type 12 genome. The map positions on the adenovirus type 12 genome for the mRNA from the cell lines as indicated correspond to regions located approximately between 0 and 0.1 and 0.74 and 0.88 fractional length units on the L strand and to regions between 0.63 and 0.74 and 0.89 and 1.0 fractional length units on the H strand. The HA12/7 line lacks mRNA complementary to the region between 0.74 and 0.88 fractional length units on the L strand. Similar data are found for the nuclear RNA, except that the regions transcribed are more extensive than those observed in mRNA. The polarity of the H strand has its 3'-end on the right terminus in the EcoRI A fragment, and the L strand has its 3'-end on the left terminus in the EcoRI C fragment. Thus, the H strand is transcribed from right to left (1 = leftward strand); and the L strand is transcribed from left to right (r = rightward strand). The designations H and L refer to the relative heavy and light densities of the two strands in polyuridylic-polyguanylic acid-CsCl density gradients. The EcoRI C-H and D-H complements have been shown to be part of the intact L strand; thus, there is a "reversal in heaviness" on the left terminus of the viral DNA.

Intracellular forms of adenovirus DNA. V. Viral DNA sequences in hamster cells abortively infected and transformed with human adenovirus type 12

The persistence of viral DNA in BHK-21 cells abortively infected with human adenovirus type 12 has been investigated using reassociation kinetics. No indication of an increase in the amount of viral DNA per cell has been found. On the contrary, the amount of intracellular viral DNA sequences decreases rapidly after infection. Thus, free adenovirus type 12 DNA does not replicate in BHK-21 cells. The influence of the multiplicity of infection on the amount of persisting adenovirus type 12 DNA has also been explored. The viral DNA sequences persisting in four lines of hamster cells transformed in vitro by adenovirus type 12 at various multiplicities of infection have been quantitated and mapped by reassociation kinetics experiments using restriction endonuclease fragments of 3H-labeled adenovirus type 12 DNA. All the EcoRI restriction nuclease fragments of the adenovirus type 12 genome are represented in each of the four cell lines. Individual fragments of the viral genome are represented in multiple copies in non-equimolar amounts.

mRNA from the transforming segment of the adenovirus 2 genome in productively infected and transformed cells

We have identified two mRNA species transcribed from the adenovirus 2 genome section (HindIII-G fragment) believed to harbor genes for initiation and maintenance of cell transformation. The HindIII-G fragment occupies the left 7.5% of the genome and is transcribed from left to right [poly(U:G) r strand]. Poly(A)-terminated labeled mRNA was isolated from polyribosomes of adenovirus 2 early infected KB cells and from the transformed cell line 8617, hybridization purified using the HindIII-G fragment, and electrophoresed on formamide-polyacrylamide gels. Viral mRNA's of 24S (1.2 X 10(6) daltons) and 14S (4.5 X 10(5) daltons) were isolated from early infected cells and of 22S (1.0 X 10(6) daltons) and 14S from 8617 cells. Hybridization competition indicated that HindIII-G-specific mRNA was present in the polysomes at one-sixth the concentration late after infection as compared with early, indicating that the proteins coded by the transforming segment may be synthesized at reduced amounts during late stages. Only 1/10 the amount of RNA labeled late annealed to the G fragment as compared with that labeled early (per weight of RNA). Thus, synthesis of transforming gene mRNA is probably "turned off" late after infection. Both 24S (22S) and 14S mRNA's from infected and 8617 cells were complementary to the Hpa I-E fragment (left 4.1% of genome). The Hpa I-E fragment is too small to encode 24S and 14S species, which implies that the 5'-terminal regions of both species are coded by the same DNA sequences.