Table 3. Relative amounts of mutant FMDV RNA released from cells ^a.

Electroporated FMDV transcript b	Viral RNA released (%) c	Calculated number of infectious particles released per cell d	Plaque-forming capacity e
WT	100±48	34±30	Yes
WT R55W	356±65	121	Yes
V19–4	7±1	2	No
V19–4 R55W	14±10	5	At 72 hpt
V15–9	5±1	2	No
V15–9 R55W	32±15	11	Yes
V3	9±5	3	No
V3 R55W	79±30	27	Yes

^a10⁶ BHK–21 cells were electroporated with 8 µg of FMDV RNA, and the amount of extracellular FMDV RNA at 6 hours post–electroporation was quantitated as detailed in Materials and Methods.

^bMutant FMDVs analysed. Their construction is described in Materials and Methods, and depicted in Figure 1; R55W indicates the presence of replacement R55W in 2C.

^cViral RNA released per cell for the indicated transcripts. Values are expressed as the percentage of the amount of RNA released per cell in the transfections with FMDV WT. Each value is the average of at least three determinations; standard deviations are given. Relative viral RNA released per cell in mutant FMDVs defective in plaque–forming capacity is below 20% of FMDV WT (calculated from data shown in Figure 5).

^dCalculated number of infectious particles released per cell for wild type and mutant FMDVs. An average number of 34 PFU are released per cell in infections with FMDV WT (data in Figure 3). The calculation assumes that one FMDV WT infectious particle is equivalent to one PFU, and that the number of infectious particles released by each mutant virus correlates with the viral RNA released.

^ePlaque–forming capacity of mutant FMDV transcripts (data in Figure 4). Yes indicates the ability of mutant transcripts to develop plaques in 48 h. No indicates that no plaques were observed at 72 hours post–electroporation. Plaque development with V19–4 R55W was only observed at 72 hours post–electroporation. Procedures for plaque assays of viral transcripts are detailed in Materials and Methods.