Construction of plasmids for PPRV rescue. (A) The cDNA fragments F1, F2, F3 and F4 were reverse transcribed and amplified from PPRV/N75/1 genomic RNA. The hammerhead ribozyme sequence (HamRz) and the hepatitis delta virus ribozyme sequence (HdvRz) were introduced to the 5′ end of F1 and the 3′ end of F4, respectively. All fragments were then subcloned stepwise into the pCI vector to produce plasmid pN75/1. (B) DNA fragments Fa (from the HamRz to the GS sequence of M with a Not I site introduced at 3′ end) and Fb (from GE of P gene to the end of F1 with Not I and Pme I sites introduced at the 5′ end) were PCR-amplified from pN75/1 and ligated together to get fragment Fab, then section F1 of pN75/1 was replaced with Fab to get plasmid pN75/1-insertion. The net result was equal to insertion of a morbillivirus gene start (GS) sequence, Not I and Pme I sites, gene end (GE) sequence and CTT intergenic trinucleotide into pN75/1 between nt 3405 and 3406 of the PPRV/N75/1 genome cDNA sequence. (C) The GFP ORF with a Kozak sequence at the 5′ end of the ORF was inserted into plasmid pN75/1-insertion to produce plasmid pN75/1-GFP.