The murine AIDS (MAIDS) virus has a unique sequence in the gag p12 region, which could be responsible for MAIDS development. RNA preparations from the spleens of normal uninfected C57BL/6 mice contain a transcript hybridizing with this sequence. Levels of the transcript in the kidney of C57BL/6 mice were higher than in the spleen, liver or thymus. Although BALB/c, NFS, DBA/2 and SL murine strains also contained genomic sequences hybridizing with the MAIDS virus-specific probe, no transcript hybridizing with the probe was detected in these strains of mice. The cDNAs carrying the transcript expressed in C57BL/6 mice were molecularly cloned. The complete nucleotide sequence of the clone indicates that the transcript is one of the endogenous murine leukaemia virus-related sequences containimg large deletions from the R and U5 regions of the 5' long terminal repeat (LTR) to gag p15, from the C-terminal region of pol p40 (integrase) to the N-terminal region of env p15E, and many short deletions in the 3' LTR U3 region. The nucleotide sequence in the gag p12 region of the transcript was closely similar to that of the MAIDS virus, but the amino acid sequence was less similar because of frameshifting, even when translated. As the MAIDS virus was isolated from C57BL/6 mice with radiation-induced leukaemia, this transcript may be the progenitor of the MAIDS virus. To determine whether the gag p12 region of the transcript contains a functional sequence, a recombinant virus was generated by replacing the gag p12 region of a replication-competent BM5eco virus with that of the endogenous transcript. The recombinant virus was replication-competent, and the p12 region of the transcript retained the functional sequence present in the BM5eco virus.
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