Hybrid viruses between human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus strain mac (SIVMAC) are invaluable to various fields of HIV-1 research. To date, however, no replication-competent HIV-1 strain containing the gag capsid (CA) region of SIVMAC has been reported. To obtain the viable gag gene chimeric virus in an HIV-1 background, seven HIV-1 strains carrying a part of SIVMAC CA or a small deletion in the CA region were constructed and examined for their biological and biochemical characteristics. While all the recombinants and mutants were found to express Gag and to produce progeny virions on transfection, only one chimeric virus, which has 18 bp of SIV gag CA sequence in place of the region encoding the HIV-1 CA cyclophilin A (CyPA)-binding loop, was infectious for human cell lines. Although this chimeric virus was unable to grow in monkey lymphocytic cells like wild-type (wt) HIV-1 did, it grew much better than wt virus in the presence of cyclosporin A in a human cell line which supports HIV-1 replication in a CyPA-dependent manner. These results indicate that the transfer of a small portion of the SIVMAC CA region to HIV-1 could confer the CyPA-independent replication potential of SIVMAC on the virus
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