TABLE 1.

Plaque phenotype of Arg183 mutant viruses

Virus	30°C titer (109 PFU/ml)	EOP (40°C PFU/ ml/30°C PFU/ml)	40°C plaque sizea (mm)
SIN HR	4.4	0.91	5
ts6	1.7	0.0016	5
ts11	8.3	0.00013	5
Ser183	3.9	0.03	<1
Ala183	6.1	0.11	<1
Lys183	2.3	0.48	2

^aWhen infected at time zero or shifted to 40°C at 1 h p.i., ts6 and ts11 do not form plaques even after 2 days of incubation. However, revertants arise at low dilutions and are wild type in plaque size (5 mm).