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Member viruses of the genus Respirovirus possess hemagglutinin and neuraminidase activities in their receptor binding protein (RBP). These viruses have six transcriptional elements. Unedited P mRNA encodes the phosphoprotein (P), whereas insertion of a G nucleotide in P mRNA transcripts provides access to the V ORF except in the case of human parainfluenza virus 1 (HPIV-1). All members encode a non-structural protein (C) from a separate ORF in the P/V mRNAs. Amino acid sequence relatedness within the genus ranges from low to high, depending on the protein, and is always higher than in comparisons with members of other genera. For example, within respiroviruses, the HPIV-1 nucleocapsid protein (N) is 88% identical to that of Sendai virus (SenV), its “murine counterpart”, and 63% identical compared to that of human parainfluenza virus 3 (HPIV-3).
See discussion under family description.
HPIV-1 and HPIV-3 are significant agents of respiratory tract disease. They represent distinct serotypes defined by a lack of significant cross-neutralization and cross protection. Their gene maps are similar but not identical (editing by HPIV-3 results in a unique D protein not seen in other members of the Paramyxoviridae, but does not access the V coding sequence, whereas HPIV-1 is the only member of the subfamily that lacks editing). They share low to high sequence relatedness among the various proteins (see above). SenV, a close relative of HPIV-1, is found predominantly as a pathogen of laboratory mice. However, this virus was isolated from the lungs of infants during a fatal epidemic of newborn pneumonitis in Japan in 1952. SenV is distinguished from HPIV-1 by host range: specifically, HPIV-1 is permissive and pathogenic in humans whereas in mice it grows poorly or not at all and is nonpathogenic. Conversely, SenV is highly permissive, transmissible and pathogenic for laboratory mice, but it is not found in wild caught mice. The two viruses have considerable sequence relatedness (see above) and antigenic similarity but can also be clearly distinguished on either basis; also, HPIV-1 lacks editing and a V protein. Bovine parainfluenza virus 3 (BPIV-3) is a close relative of HPIV-3, but they differ in their host ranges, which overlap but exhibit specificity. For example, in humans, HPIV-3 replicates efficiently, is easily transmissible and causes disease, whereas BPIV-3 is highly attenuated, nonpathogenic and poorly transmissible. HPIV-3 and BPIV-3 exhibit considerable genetic and antigenic similarity but can also be clearly distinguished on either basis. For example, HPIV-3 and BPIV-3 are 25% related antigenically by reciprocal cross-neutralization and hemagglutination inhibition studies. Also, BPIV-3 expresses a V protein whereas it is unclear whether HPIV-3 does. HPIV-3 is a hemagglutinating virus that was recovered from the mouth of a blue monkey (Cercopithecus mitis Wolf, 1822).
Species demarcation is now entirely based on the distance in the phylogenic tree of complete large (L) protein amino acid sequences. Since the primary criterion is tree topology, whether a virus belongs to the same species becomes a matter of branch length between the nearest node and the tip of the branch. This length is defined as 0.03 in the trees generated as described in the legend to Figure 3.Paramyxoviridae.
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