bands seen in the DNA size marker, were marked with a ball -point pen at the places where small holes had been pierced in the gel earlier ( see above ). Thus it was easy to monitor the size of the fragments showing hybridisation to the probe. The gel was then peeled off and the membrane w~shed in 6 X sse with gentle rocking for 10 minutes to wash away any residual agarose sticking to the membrane. After air drying at room temperature, the membrane was baked at so0e for two hours. The baked filter was stored at room temperature in a dessicator, if not used immediately. The dehydrated gel was restained in water containing 0.5 ug I ml ethidium bromide for 30 minutes and examined on a short wave UV transilluminator to check for the presence of any DNA fragments that escaped blotting. The absence of any residual bands indicated that the transfer was complete.

Restriction fragments of DNA resolved on agarose gel were transferred to nylon membrane ( GeneScreen or GeneScreen Plus by the capillary blotting procedure of Southern ( 1975 ) as described by Maniatis et al., ( 1982 ) . After the completion of electrophoresis, the gel was stained and photographed as described earlier. Position of the various bands obtained in the DNA size marker lane were marked by piercing small holes at the two ends of each band in the gel with a yellow tip. The gel was then denatured, neutralised and blotted essentially as described by Maniatis et al., ( 1982 ) . Locally available coarse absorbent paper was used to make the paper towels of the appropriate size. In case of genomic DNA from mammalian cells, the agarose gel was first treated with 0.25 M HCl for 10 minutes, followed by the rest of the procedure as mentioned above. The transfer buffer was 20 X SSPE in all cases. To prevent the absorption of fluid from the 3 MM paper under the gel directly to the blotting paper atop the nylon membrane, the gel was surrounded with polythene sheets to minimise the direct contact between the blotting paper and the 3 MM paper placed under the gel. The blotting was performed for 18 -24 hours. After the transfer was over, the paper towels and the 3 MM papers on top of the nylon filter were peeled off. The gel along with the attached membrane, was turned over and kept on a clean sheet of 3 MM paper with the gel side up. The position of the gel slots was marked with a ball -point pen. Also, the positions of the

southern blot.

albumin (BSA) in PBS for 2 hat 4°C. For detection of r-bmZPI, a murine monoclonal antibody (MAb), MA-813, generated against E. coli expressed r-bmZP1 (Govind et al., 2000), was used as the primary antibody. The cells were incubated with 1 :500 dilution of MA-813 ascites fluid for 2 hat 4°C. Cells were washed 5 times with PBS and incubated for 1 h with a 1:800 dilution of goat anti-mouse Ig-fluorescein isothiocyanate (FITC) conjugate (Sigma) at 4°C. After washing with PBS, coverslips with the cells were mounted in glycerol : PBS (9 : 1 ), and examined under an Optiphot fluorescent microscope (Nikon, Chiyoda-Ku, Tokyo, Japan). For detecting r-dZP3, MAb, MA-451 (1 :500 dilution of ascites fluid), generated against porcine ZP3f3 (a homologue of dZP3) and immunlogically cross-reactive with dZP3 (Santhanam et al., 1998) was used. For detecting r-rG, rabbit polyclonal antibodies (1:1000 dilution) against E. coli expressed r-rG, was used as primary antibody. The polyclonal antibody was provided by Dr. Sangeeta Choudhury, Project Associate, Gamete Antigen Laboratory, National Institute of Immunology, New Delhi. Goat anti-mouse immunoglobulins-FITC conjugate (1 :800) and goat anti-rabbit immunoglobulins-FITC conjugate (1 :2000; Pierce) were used for detecting anti-dZP3 and anti-rG antibodies respectively

Initial standardization of transfection conditions was done using VRbmZPl plasmid DNA and COS-I mammalian cell line. In brief, cells were cultured in T-25 tissue culture flasks in Dulbecco's Modified Eagle's Medium (DMEM) supplemented with 10% fetal calf serum (FCS) at 37°C with 5% C02. For subculturing, cells were trypsinized (0.5% trypsin + 0.2% EDTA in DMEM without FCS), centrifuged at 250 X g for 10 min, resuspended in DMEM supplemented with 10% FCS and aliquoted into T-25 flasks. For transfection, cells were seeded on coverslips in a 24-well tissue culture plate at a density of 5x 104 cells/well, a day prior to transfection. To standardize in vitro transfection conditions for optimum expression of bmZP1, varying amount of plasmid DNA was mixed with lipofectamine in DMEM devoid ofFCS (final reaction volume 200 f.!l) and incubated at RT for 45 min. The cells on the coverslips were washed twice with plain DMEM devoid of FCS. DNA-Iipofectamine complex was added dropwise to the cells and the plate incubated for 8 h at 3 7°C in humidified atmosphere of 5% C02• Subsequently, 1 ml of DMEM containing 10% FCS was added per well and cells allowed to grow for 48 h. After incubation, cells were processed for visualization of r-bmZPl by indirect immunofluorescence assay. Cells were washed twice with phosphate buffer saline (PBS; 50 mM Phosphate and 150 mM NaCI, pH 7.4), fixed in chilled methanol (-20°C) for 3 min and blocked with 3% bovine serum

Detection of the expressed recombinant protein following i11 vitro transfection of mammalian cells with the plasmid DNA.