Semliki Forest virus (SFV) is an enveloped alphavirus that is transmitted in the wild by mosquito vectors. In tissue culture cells, SFV requires cholesterol in the cell membrane both for virus membrane fusion and for the efficient exit of progeny virus from the cell. A previously isolated SFV mutant, srf-3, is strikingly less cholesterol-dependent for virus fusion, exit, and growth due to a single amine acid change in the E1 spike protein subunit, proline 226 to serine. Here we show that when mosquitoes were infected by intrathoracic injection at a range of virus multiplicities, the growth of srf-3 was significantly more rapid than that of wild-type virus, particularly at low multiplicity infection. The differential cholesterol requirements for wild-type and srf-3 infection were maintained during virus passage through mosquitoes. The presence or absence of cholesterol in the srf-3 virus membrane did not affect its infection properties in mosquitoes. Thus the srf-3 mutation causes a growth advantage in the tissues of the mosquito host.
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