The two open reading frames of the 22K mRNA of human respiratory syncytial virus: sequence comparison of antigenic subgroups A and B and expression in vitro.

The sequence of the 22K mRNA of strain 18537 of antigenic subgroup B of human respiratory syncytial virus (RSV) was determined by sequencing cloned cDNAs of intracellular mRNA. Comparison with the corresponding sequence of the A2 strain of subgroup A showed that there is 78% nucleotide sequence identity overall, that the amino acid sequence of the 22K protein is 92% identical between subgroups and that the 22K mRNA of both subgroups contains a second, internal, overlapping open reading frame (ORF) whose length, nucleotide sequence and potential translational start and stop sites were highly conserved and whose predicted product has 62% amino acid identity between subgroups. Sequence analysis of 36 cDNAs of intracellular 22K mRNA of strain A2 did not detect nucleotide insertions or deletions in the region of overlap between the two ORFs, indicating that the majority of intracellular 22K mRNA is a faithful copy containing the two distinct ORFs. Translation in vitro of mRNAs transcribed from engineered cDNAs showed that an mRNA which contained only the second, internal ORF directed the synthesis of a previously unidentified polypeptide of the predicted size, whereas mRNA representing the complete gene directed the synthesis of both the 22K protein and the product of the internal ORF. This latter species was synthesized in vitro as a discrete, separate protein rather than as a fusion protein. It is not yet known whether this protein is synthesized in RSV-infected cells.