TABLE 4.

The 530 and 1030 mutations independently confer the ts and att phenotypes

Virusa	Presence (+) or absence (−) of the following mutation:			Mean virus titer (log10 PFU/ml) at temp (°C) ofb:						Shutoff temp (°C)c	Mean ± SE titer (log10 PFU/g) ind:
	cp	530	1030	32	35	36	37	38	39		Nasal turbinates	Lungs
A2 (wt) (HEK-7)	−	−	−	4.7	4.6	4.4	4.4	4.5	4.3	>39	4.4 ± 0.07	5.0 ± 0.06
rA2cp	+	−	−	6.0	5.9	6.0	5.7	5.6	4.8e	>39	3.8 ± 0.11	4.1 ± 0.10
rA2cp530	+	+	−	5.5	5.4	5.2	4.9	4.8e	<0.7	39	2.4 ± 0.12	3.9 ± 0.05
cpts530	+	+	−	5.5	5.4	5.4	4.9	5.2e	<0.7	39	3.3 ± 0.04	4.2 ± 0.04
rA2cp1030	+	−	+	5.5	5.6	5.3	4.6e	2.2e	<0.7	38	2.4 ± 0.19	2.9 ± 0.07
rA2cp530/1030	+	+	+	5.6	4.7e	<0.7	<0.7	<0.7	<0.7	36	<2.0 ± 0.00	<1.7 ± 0.00
cpts530/1030	+	+	+	4.5f	3.5e	<0.7	<0.7	<0.7	<0.7	36	<2.0 ± 0.00	<1.7 ± 0.00

^aRecombinant (r) viruses each contained the “HEK” F gene mutations and the six silent L gene “sites” mutations (26). A2 (wt) (HEK-7), cpts530, and cpts530/1030 were biologically derived viruses. 

^bBoldfacing indicates the virus titer at the shutoff temperature. 

^cDefined as the lowest restrictive temperature at which a 100-fold or greater reduction in titer was observed. 

^dMice in groups of five were administered 10⁶ PFU of the indicated virus intranasally under light anesthesia on day 0 and sacrificed on day 4. 

^ePinpoint plaque size. 

^fSmall plaque size.