Is mutant was obtained by site directed mutagenesis using the following olignucleotides: 59CCTGTCTCTCAGTACCGCCCTTTTTCCTAG39 and 59CTTTCATTTGGCATCCTTCC39, respectively.Cell culture, transfection and virus preparationHEK293T cells were grown in DMEM medium (Dulbecco’s modified Eagle’s medium) supplemented with glutamine (2 mM),Figure 1. Primary structure of MuLV NC protein and schematic representation of the mutants used here. Numbers indicate amino acid positions. The zinc finger is drawn with the Zn ion coordinated by the CCHC residues. The broken line represents the ITI-007 site deleted amino acids. doi:10.1371/journal.pone.0051534.gRoles of the NC in HIV-1 and MuLV Replicationspenicillin (100 U/mL), streptomycin (100 mg/mL) and heatinactivated fetal calf serum (10 v/v) at 37uC. Transfections were performed as previously described [35]. In a standard experiment, 3.56106 cells were grown in 10 cm dishes. The next day, 8 mg of plasmid DNA were transfected by phosphate calcium precipitation. In all cases, in order to eliminate the plasmid in excess in the medium, the cells were trypsinized 6 hours after transfection, centrifuged and transferred in a new dish. The supernatant was harvested 48h after transfection, centrifuged at 1500 rpm during 10 min and filtered at 0.45 mm. Cells were collected by pipetting with PBS and centrifuged 5 min at 1500 rpm.DNA and RNA extractionsNucleic acids extractions from virions were performed as previously described [26]. Before ultracentrifugation, 400 ml of HIV-1 mutant virions (DZF2) obtained as previously described in [26] were systematically added to MuLV supernatants as a tracer to check DNA extraction. However, no tracer was added to the supernatants during the HIV-1 or the HIV-1/MuLV coexpression assays. Then, virions were purified from 15 ml of filtered culture supernatants by centrifugation through a 20 sucrose cushion at 30 000 rpm for 1h 30 at 4uC in an SW32 rotor. Pellets were resuspended in 160 ml of DMEM with 8 U of DNase (RQ1, Promega). One aliquot of virion samples (25ml = 1/6) was saved for virion quantification by Western-Blot analysis as previously in reference [36] and the rest of virions was incubated at 37uC for 45 min to reduce contamination by the transfectingplasmid DNA. Then, 44 mL of TES 4X (200 mM Tris pH 7.5, 20 mM EDTA, 0.4 SDS) and 20 mg of tRNA carrier were added to the virions before extraction of the nucleic acids by phenol/chloroform and ethanol precipitation. DNA was extracted from cells with DNAzol (MRC) according to the manufacturer’s instructions and as previously described [26]. To avoid any contamination with viral cDNA associated with the particles, cells were extensively washed with cold PBS before DNA extraction. DNA was quantitated by measuring optical absorption at 260 nm.CTTAAGCTAGCTTGCCAAACC antisense, and for specific detection of HIV-1 multi-spliced cDNA (MS cDNA), 15755315 sHIV5967 = 59-CTATGGCAGGAAGAAGCGGAG sense and aHIV8527 = 59-CAAGCGGTGGTAGCTGAAGAG antisense. A standard curve was generated from 50 to 500 000 copies of pRR88-wt plasmid. For each experiment, the DNA purified from virions was checked by a q-PCR assay using the HIV primer pairs (sHIV5967/aHIV8527) specific for the HIV-1 multispliced cDNA forms as previously described [26] to monitor the viral DNA contained in the HIV-1 virions added as tracer. Systematically, cellular GAPDH gene level was determined for standardization of the cellular DNA samples. The background measured from the transfected pRR88 plasmid.