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A family of related sequences that includes approximately 500,000 members is the most prominent short dispersed repeat family in primate and rodent DNA's. The primate sequence is approximately 300 base pairs in length and is composed of two imperfectly repeated monomer units, whereas the rodent repeat consists of only a single monomer. Properties of this repeat sequence, its flanking sequences in chromosomal DNA, and RNA's transcribed from it suggest that it may be a mobile DNA element inserted at hundreds of thousands of different chromosomal locations.

Poly(A)-containing HeLa cell mRNA prepared from cells labeled with [3Plphosphate was found to contain a variety of methylated, blocked 5'-terminal structures of two general types: m7GpppNm-Np and m7GpppNm-Nm-Np. In addition, about one-third of the [3Hlmethyl label was present in the N6-methyladenosine; this labeled nucleoside was not found in the 3'-terminal one-third of the mRNA chain and thus may also be in the 5' portion of the mRNA.The 5' end of a variety of viral mRNA molecules consists of an unusual type of methylated oligonucleotide with the general structure m7G(5')ppp(5')Nm-Np (1-6). Because the addition through pyrophosphate linkage of the terminal m7G renders the terminal dinucleotide resistant to digestion by the usual ribonucleases, the structure has been called a "cap" (7). Perry and Kelley (8) have shown that mammalian cell mRNA contains methyl groups, and, with the availability of methods used to characterize the cap structures in viral mRNAs, we have examined HeLa cell mRNA for the presence of caps. As pointed out by Rottman et al. (7), information about various steps of methylation of potential mRNA precursors by the cell should aid in understanding eukaryotic mRNA formation. MATERIALS AND METHODSRadioactive Labeling. Growth of suspension cultures of HeLa cells was in Eagle's medium. Cells were labeled with 32p for 4 hr in phosphate-free medium as described (9). Labeling with [methyl-3H]

DNA base sequence comparisons demonstrate that the principal family of 300-nucleotide interspersed human DNA'sequences, the repetitive double-strand regions of HeLa cell heterogeneous nuclear RNA, and specific RNA polymerase III in vitro transcripts of cloned human DNA sequences are all representatives of a closely related family of sequences. A segment of approximately 30 residues of these sequences is highly conserved in mammalian evolution because it is also present in the interspersed repeated DNA sequences of Chinese hamsters. Further DNA sequence comparisons demonstrate that a portion of this highly conserved segment of repetitive mammalian DNA sequence is similar to a sequence found within a low molecular weight RNA that hydrogen-bonds' to poly(A) terminated RNA molecules of Chinese hamsters and a sequence that forms half of a perfect inverted repeat near the origin of DNA replication in papovaviruses.

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A consensus sequence has been determined for a major interspersed deoxyribonucleic acid repeat in the genome of Chinese hamster ovary cells (CHO cells). This sequence is extensively homologous to (i) the human Alu sequence (P. L. Deininger et al., J. Mol. Biol., in press), (ii) the mouse B1 interspersed repetitious sequence (Krayev et al., Nucleic Acids Res. 8:1201-1215, 1980) (iii) an interspersed repetitious sequence from African green monkey deoxyribonucleic acid (Dhruva et al., Proc. Natl. Acad. Sci. U.S.A. 77:4514-4518, 1980) and (iv) the CHO and mouse 4.5S ribonucleic acid (this report; F. Harada and N. Kato, Nucleic Acids Res. 8:1273-1285, 1980). Because the CHO consensus sequence shows significant homology to the human Alu sequence it is termed the CHO Alu-equivalent sequence. A conserved structure surrounding CHO Alu-equivalent family members can be recognized. It is similar to that surrounding the human Alu and the mouse B1 sequences, and is represented as follows: direct repeat-CHO-Alu-A-rich sequence-direct repeat. A composite interspersed repetitious sequence has been identified. Its structure is represented as follows: direct repeat-residue 47 to 107 of CHO-Alu-non-Alu repetitious sequence-A-rich sequence-direct repeat. Because the Alu flanking sequences resemble those that flank known transposable elements, we think it likely that the Alu sequence dispersed throughout the mammalian genome by transposition.