Unfolding of the viral cycle 1: transcription

During transcription, the required virus multiplication cycle is initiated, leading to the synthesis of messenger RNA.

• The RNA-dependent RNA polymerase recognizes the leader, binds to the 3' end of the virus genome at the level of the genomic promoter, and initiates transcription of the coding sequence of the first gene, the N nucleoprotein.

• When it reaches the termination signal, it releases the synthesized messenger RNA. It then reinitiates the transcription of the next gene, located 3 nucleotides (CUU for PPRV) away from the intergenic region, and continues like this in a sequential manner up to the L gene.

• However, at each intergenic sequence, its reinitiation frequency drops, leading to a decreasing gradient (called the transcription gradient) in the amount of messenger RNA produced. In other words, the intergenic sequences are “attenuated”. There is a greater abundance of messenger RNA of the first gene, N, than of the last gene, L. This mechanism is a form of regulation aiming to produce the right proportion of each protein for the future virions.

• Each messenger RNA is translated into a protein by the ribosomes of the infected cell. Once produced, the viral proteins migrate towards cellular organelles (endoplasmic reticulum and Golgi apparatus), then H and F steer themselves towards the plasma membrane.

When sufficient viral N and P proteins have accumulated, transcription gradually gives way to replication.