We have determined the nucleotide sequences of long terminal repeat (LTR) regions of Syrian hamster intracisternal A particle (IAP) genes. The size of the LTRs was 350 base-pairs (bp) and 376 bp in two clones, H10 and H18, respectively. Two LTRs at both ends of the IAP gene were linked to directly repeating 6 bp hamster sequences. Many structural features common to the integrated retroviral LTRs such as "CAT" box, "TATAA" box, polyadenylation signal, and terminal inverted repeat (3 bp), were present on each LTR. The estimated length of R region (about 60 bp) was similar to that of the murine leukemia-sarcoma virus. In contrast, the calculated U5 region of 54 bp was the shortest among those of the retroviruses so far studied. Furthermore, from the analysis of primer binding sites, phenylalanine tRNA was for the first time identified as a presumed primer tRNA for reverse transcription. These results clearly distinguish Syrian hamster IAP LTRs from other retroviral ones. Based on the comparison of the sequences between Syrian hamster and laboratory mouse LTRs, the structural features peculiar to the IAP LTRs and the origin of the IAP genes are discussed.

We isolated DNA clones of intracisternal A-particle (IAP) genes from the genome of an Asian wild mouse, Mus caroli. A typical M. caroli IAP gene was 6.5 kilobase pairs in length and had long terminal repeat (LTR) sequences at both ends. The size of the LTR was 345 base pairs in clone L20, and two LTRs at both ends of this clone were linked to directly repeating cellular sequences of 6 base pairs. Each LTR possessed most of the structural features commonly associated with the retrovirus LTR. The restriction map of the M. caroli IAP gene resembled that of Mus musculus, although the M. caroli IAP gene was 0.4 kilobase pairs shorter than the M. miusculius IAP gene in two regions. Sequence homology between the M. caroli and M. musculus IAP LTRs was calculated as about 80%, whereas the LTR sequence of the Syrian hamster IAP gene was about 60% homologous to the M. caroli LTR. The reiteration frequency of the M. caroli IAP genes was estimated as 200 to 400 copies per haploid genome, which is at least 10 times the reported value. These results suggest that the IAP genes observed in the genus Miis are present in multiple copies with structures closely resembling the integrated retrovirus gene. 0.15 M NaCl plus 0.015 M sodium citrate) and 0.1% sodium dodecyl sulfate at 65°C.Construction and screening of gene libraries. The gene libraries of M. caroli and M. cerv'icolor kidney DNAs were prepared as described previously (20,25). The 13to 17-kb DNAs prepared by size fractionation on a sucrose gradient 352 on July 7, 2020 by guest

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