Page 53 - Avian Virology: Current Research and Future Trends

46 | Samal

HN
F

Envelope

M

P
L

N + RNA

Figure 2.2 Schematic diagram of a Newcastle disease virus particle. The lipid bilayer derived from host cell membrane is shown as envelope.
The viral matrix protein is shown as a black concentric circle beneath the envelope. The haemagglutinin-neuraminidase (HN) and the fusion
Figure 2
(F) glycoproteins are shown imbedded inside the viral envelope. The inside of virion is made up of negative-strand RNA encapsidated with
nucleocapsid protein (N) and associated with the phosphoprotein (P) and the large polymerase protein (L).

line (DF-1), African green monkey kidney cell line (Vero) and of a 12-nt insert in the open reading frame (ORF) of P and V
baby hamster kidney cell line (BHK-21). However, propagation proteins after genome position 2381. The addition of 12-nt did
of low virulence NDV strains requires supplementation of 10% not change the reading frames of P and V proteins but increased
fresh allantoic fluid or 1.0 µg/ml trypsin in the medium. Typi- their lengths by 4 aa. The sequence of the 12-nt insert also varies
cal cytopathic effect consists of multinucleated cells (syncytia), among NDV strains. Additionally, NDV strains isolated in West
or plaques if an overlay medium is used. Generally, the virus Africa were found to have genome length of 15,198 nt, in which
titre obtained in cell culture system is 1–2 log lower than that there was a 6-nt insert in the N gene and another 6-nt insert in
10
obtained in ECE. the intergenic sequence between HN and L genes (Kim et al.,
2012; Samuel et al., 2013). The 15,192 and 15,198 nt genome
length viruses were isolated after 1960. However, an NDV strain
Genome organization, transcription and isolated in 1946 was found to have a genome length of 15,192 nt,
RNA replication indicating that these viruses were probably circulating prior to
1960 (Qiu et al., 2011). The evolutionary significance of these
Genome length different genome length viruses is not known. Currently, all three
The genome of NDV is a non-segmented, linear, negative-sense, genome-length viruses are circulating in the nature but 15,192 nt
single-stranded RNA (Fig. 2.3A). Three different lengths of viruses are more prevalent than 15,186 nt and 15,198 nt viruses.
the NDV genome have been reported. The initial NDV strains The sequence alignment and phylogenetic tree analysis revealed
sequenced were 15,186 nucleotides (nt) long and these strains that NDV strains which belonged to a genome length clustered
were isolated before 1960 (Krishnamurthy and Samal, 1998; de together (Paldurai et al., 2014a) and circulate in a location (Kim
Leeuw and Peeters, 1999; Römer-Oberdörfer et al., 1999). A et al., 2012). The biological significance of the 6-nt N and 12-nt P
second genome size category of 15,192 nt was first found in an iso- inserts has been investigated (Paldurai et al., 2014a). The results
late from geese in China (Huang, Y. et al., 2004). The increase in showed that the addition of the 6-nt N or the 12-nt P insert into
length was due to presence of a 6-nt insert in the 5′ (downstream) the genome of 15,186-nt length viruses or the deletion of the 6-nt
non-coding region of the N gene after genome position 1647. insert from a 15,192-nt genome virus resulted in slight growth
Sequence analysis showed that the 6-nt insert is present in many retardation and in a decrease of virulence. These results suggest
other NDV strains at the same genome position, but the sequence that the 6-nt and the 12-nt inserts found in the 15,192-nt and
of the insert varies among NDV strains (Zou et al., 2005; Ujvári, 15,198-nt genome-length viruses may not have any direct role in
2006). A third genome size category of 15,198 nt was first recog- replication and virulence, but they make up the optimal genome-
nized in an avirulent isolate from ducklings in Germany in 1999 length that is required for efficient replication of that virus strain
(Czeglédi et al., 2006). The increase in length was due to presence (Paldurai et al., 2014a).