ISOLATION AND PRELIMINARY CHARACTERIZATION OF 25 TEMPERATURE-SENSITIVE MUTANTS OF MOUSE CYTOMEGALOVIRUS

To study the pathogenicity of mouse cytomegalovirus (MCMV) and to iden tify virulence determinants, we have isolated and phenotypically chara cterised 25 temperature-sensitive (ts) mutants. Six of these (tsm9, ts m13, tsm20, tsm22, tsm28 and tsm30) failed to replicate in mice and we re 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 85,00 0 fold less virulent than wt. One mutant (tsm28) did not plaque or rep licate 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 r eplicate 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, tsm2 2 and tsm30) failed to form plaques at 40 degrees C and were severely restricted in their replication at 40 degrees C. The remaining 11 muta nts exhibited varying degrees of restriction in ability to plaque and/ or replicate at non-permissive temperatures. These 25 mutants, togethe r with 6 isolated previously, comprise at least 24 complementation gro ups.