REPLICATION, TRANSCRIPTION, AND TRANSLATION:

• Viral replication takes place in the cytoplasm
• Paramyxoviridae genomes contain 6-10 genes.  (Genera Paramyxo and Morbilli contain six; Genus Rubula, seven; Genus Pneumo has 10 genes)
• The red strand is the (-)ss RNA which is transcribed into 6-10 subgenomic monocistronic mRNAs.
• After the mRNAs are transcribed by the transcriptase, they are translated into the viral proteins.

Figure courtesy of Dr. Alan Cann, University of Leicester web page

• When the concentration of NP builds up in the cell, the virus switches from transcription to replication.
• First, the -strand (red) is transcribed by ignoring all stop sequences and making a full-length complementary copy of +RNA (blue).
• The resulting +RNA (blue) is not mRNA but a replication template for making new -RNA (green).
• This new -RNA (green) associates with the NP and transcriptase to form the nucleocapsid and then the M, F, and HN proteins migrate to the surface of the plasma membrane.
• Paramyxoviruses bud off of the cell membrane, leaving them enveloped.
• F protein must be cleaved into two subunits in order to make the virus infectious.  An activated fusion protein evades the immune system by causing the infected cell to fuse with non-infected cells.  Many fused cells are referred to as syncytia or multi-nucleated giant cells. (Syncytia are one way to diagnose paramyxoviruses in the laboratory.)
• An interesting note about transcription of Morbilliviruses and RSV--The P gene, which is near the 3' end, has overlapping reading frames and therefore can code more than one protein.  This increases the gene coding potential.  For example, to code four proteins from the P gene, two mechanisms are used.  First is an internal initiation of translation and second is the insertion of G residues into the mRNA.  This G insertion causes a shift in the open reading frame so that the C and V proteins can be translated.