FIG. 4.

Infection of NIH 3T3 cells (A) and 293 mCAT1 cells (B) by X-MLV, X-MLV (env ΔH7), and X-MLV (env ΔRBD) was measured in the presence (100 nM) or absence of soluble Fr-RBD. The NIH 3T3 cells (CL-13) used in this experiment express fourfold more mCAT1 than normal NIH 3T3 cells (12). It should be noted that the deletion of the conserved histidine residue (His7) in X-MLV reduces the titer of X-MLV by 10-fold to 10⁴ IU/ml compared to a 10⁴-fold reduction in titer caused by the deletion of the equivalent His residue in A-MLV or Fr-MLV. +, present; −, absent.