Isolation and preliminary characterisation of twenty-five temperature-sensitive mutants of mouse cytomegalovirus

To study the pathogenicity of mouse cytomegalovirus (MCMV) and to identify virulence determinants, we have isolated and phenotypically characterised 25 temperature-sensitive (ts) mutants. Six of these (tsm9, tsm13, tsm20, tsm22, tsm28 and tsm30) failed to replicate in mice and were avirulent. Five mutants (tsm14, tsm18, tsm19, tsm25 and tsm27) were of similar virulence to the parental wild-type (wt) virus, five (tsm7, tsm15, tsm24, tsm26 and tsm31) were 12-100 fold less virulent, five (tsm8, tsm12, tsm16, tsm23 and tsm29) were 150-1500 fold less virulent and four (tsm10, tsm11, tsm17 and tsm21) were between 2,000 and 85,000 fold less virulent than wt. One mutant (tsm28) did not plaque or replicate at 39 degrees C while 5 other mutants (tsm7, tsm9, tsm23, tsm24 and tsm27) also failed to plaque at 39 degrees C but only failed to replicate or replicated poorly at 40 degrees C. A further two mutants (tsm10 and tsm13) were able to plaque and replicate at 39 degrees C but not 40 degrees C. Six other mutants (tsm14, tsm15, tsm16, tsm21, tsm22 and tsm30) failed to form plaques at 40 degrees C and were severely restricted in their replication at 40 degrees C. The remaining 11 mutants exhibited varying degrees of restriction in ability to plaque and/or replicate at non-permissive temperatures. These 25 mutants, together with 6 isolated previously, comprise at least 24 complementation groups.