TABLE 1.

Viruses used in the present studies

Virusa	Amino acids at PE2 cleavage site (−5−4−3−2−1+1)	Ratio of PE2 to PE2 + E1 + E2 + Cb	BHK specific infectivity (PFU/cpm)
TR339	GRSKRS	0.006	5.7
39R1	GRSKRR	0.035	263
FD	G−−−−S	0.255	<0.01
FD1	GRSK−S	0.277	0.13
FD2	GRS−−S	0.260	0.05
39N1	GRSKRN	0.270	0.74
FDN1	G−−−−N	0.256	0.05
39L1	GRSKRL	0.179	33.5
39V1	GRSKRV	0.197	28.4
39K70c	GRSKRS	0.004	2,000
FDK70d	G−−−−S	0.243	0.51

^aViruses were derived from cDNA clones that were isogenic except for the indicated loci. 

^bRatios derived from PhosphorImager quantitation of band densities on SDS-PAGE gels. 

^cFurin cleavage site identical to TR339; contains Glu-to-Lys mutation at E2 70. 

^dFurin cleavage site deleted; contains Glu-to-Lys mutation at E2 70.