4. 
REVERSION OF MUTANT SQL-SFV 
The frequency of reversion of the SQL-SFV system to infectious virus was 
measured, as follows: Eight independently produced SQL-virus stocks were 
activated with chymotrypsin and used to infect 2.5X10 6 BHK cells on plates using 
10 4 , 10 5 , or 10 6 particles. The cells were overlayed with agarose, and plaques 
scored after two days (Table 4). Based on the numbers of plaques observed in 
plates infected with 1x10 s infectious particles, the apparent reversion frequency 
is 1.2X10- 4 . 
Table 4 
Reversion Frequency of SFV-SQL Phenotype 
Stock 1 ’ 
Number of Plaques Observed 
10 4 * 
10 5 
10 6 
Act 6 
Unact 6 
Act 
Unact 
Act 
Unact 
1 
1 
0 
14 
0 
>100 
0 
2 
0 
0 
12 
0 
>100 
0 
3 
0 
0 
8 
0 
>100 
0 
4 
2 
0 
11 
0 
>100 
0 
5 
0 
0 
8 
0 
>100 
0 
6 
0 
0 
5 
0 
>100 
0 
7 
2 
0 
19 
0 
>100 
0 
8 
3 
0 
20 
0 
>100 
0 
‘Amount of particles used for infection of 2.5'x 10 6 cells. 
“Titre of all 8 stocks were 3 x lOVml as measured by immunofluorescence of cells 
infected with activated stock. 
*1116 virus was either activated (Act) by cleavage with chymotrypsin or left uncleaved 
(Unact). 
Recombinant DNA Research, Volume 18 
[451] 
