LACK OF CORRELATION BETWEEN SENDAI VIRUS P C MESSENGER-RNA STRUCTURE AND ITS UTILIZATION OF 2 AUG START SITES FROM ALTERNATE READING FRAMES- IMPLICATIONS FOR VIRAL BICISTRONIC MESSENGER-RNAS/

The polycistronic P/C mRNA of Sendai virus encodes five proteins (C', P, C, Y1, and Y2) each of which initiates from a distinct start site. Two major proteins, P and C, are expressed in approximately equimolar amounts from two consecutive AUGs in overlapping reading frames. To be tter understand the mechanism of expression of the C protein from a do wnstream AUG, site-directed mutants of the P/C mRNA were created and e xpressed in COS1 cells. The secondary structure of the mRNA was examin ed to determine whether the mRNA structure played any role in the synt hesis of the C protein. Our results ruled out any significant involvem ent of the 5' UTR, sequence contexts, secondary structure, distance be tween the start sites, and sequences downstream to the C-AUG. However, they are consistent with the concept that the synthesis of the C prot ein is primarily dependent on the orientation of its reading frame, i. e., +1 in relation to the upstream P reading frame. The downstream rea ding frame was translated poorly when it occurred in +2 orientation in relation to the upstream reading frame. Interestingly, all the known functional bicistronic mRNAs with overlapping reading frames from cyto plasmic RNA viruses have their downstream reading frame in +1 orientat ion relative to the upstream frame. We propose that the evolutionary c onservation of the downstream reading frame in +1 orientation in these bicistronic mRNAs is important for its efficient translation.