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348 | Lupiani et al.

Table 12.3 Comparison of the genomic regions of three serotypes MDV strains: Md5, RB-1B. GA, GX0101, CVI988, SB-1, HRPS-24, and
FC126
IRL/IRS
Serotypes Strain GenBank accession number Total length (bp) TRL (bp) UL (bp) (bp) US (bp) TRS (bp)
MDV-1 (GaHV-2) Md5 AF243438.1 177,874 14,028 113,563 26,207 10,847 13,229
RB-1B EF523390.1 178,246 14,695 113,610 26,043 11,668 12,230
GA AF147806.2 174,077 12,548 113,508 24,704 11,160 12,121
GX0101 1 JX844666.1 178,101 12,758 113,572 25,441 11,695 13,134
CVI988/Rispens DQ530348.1 178,311 14,476 113,490 26,639 11,651 12,055
MDV-2 (GaHV-3) SB-1 HQ840738.1 165,994 11,943 109,744 21,290 12,910 9306
HRPS-24 AB049735.1 164,270 11,818 109,932 20,446 12,109 8619
HVT (MeHV-1) FC126 AF291866.1 159,160 5658 111,868 18,961 8617 13,303
1 MDV GX0101 strain contains a reticuloendotheliosis virus (REV) long terminal repeat (LTR) insert at a site 267 nucleotide upstream of the SORF2
gene.

and a processive heterodimeric DNA polymerase (UL30/UL42). The DNA-binding domain of MDV OBP is located within amino
The latter group includes a deoxyuridine triphosphatase (UL50), acids 528 to 841 (Wu, T.F. et al., 2001). OBP can form dimers in
a ribonucleotide reductase composed of two non-identical solution and functions as nucleoside triphosphatase, DNA heli-
subunits (UL39/UL40), a thymidine kinase with non-specific case on partially double-stranded DNA substrates, non-specific
nucleoside kinase activity (UL23), an alkaline endo-exonuclease single-strand DNA-binding protein, and cooperatively binds
(UL12), and a uracil-DNA glycosylase (UL2) (Fig. 12.1 and with other proteins at the origin of replication (Ward and Weller,
Table 12.4). Host enzymes are also required during replication, 2011). In HSV, OBP has been reported to interact with ICP8,
such as DNA polymerase, α-primase, DNA ligase I, and topoi- UL8, and UL42 to bind and unwind duplex DNA at the origins of
somerase II (Boehmer and Lehman, 1997). replication (Ward and Weller, 2011).
After the virus enters the cell and the viral genome is injected
into the nucleus, the linear viral genome circularizes and genome Major single-strand DNA-binding protein (MDV042/
replication proceeds in two phases: theta replication and sigma UL29, ICP8)
or rolling-circle mode of replication, resulting in the formation of ICP8, encoded by the MDV042 or UL29 gene, originally known
concatemers which are essential for DNA encapsidation. as the major HSV single-strand DNA-binding protein (SSB or
The MDV-1 genome contains two cis-acting elements that MDBP), is a 1191 amino acid long protein with an expected
function as origins of DNA replication. They are located in the molecular weight of 130 kDa (Kato et al., 1999). ICP8 is involved
intron of pp14 in the terminal repeats (TRL and IRL). The 90-bp in viral DNA synthesis, control of viral gene expression, and the
MDV origin of replication shares 72% identity with the lytic formation of prereplicative sites and replication compartments
origin (oriS) of HSV-1, and it is arranged as a palindrome cen- (Ward and Weller, 2011). ICP8 has been reported to interact with
tred around an A/T-rich region (Camp et al., 1991) flanked by many viral genes, such as UL9, polymerase, helicase/primase,
three recognition sites for the origin-binding protein (Weller and UL12, ICP4 and ICP27 (Ward and Weller, 2011).
Coen, 2012). A 9-bp motif (5′CGTTCGCAC3’) in this sequence
is highly conserved among alphaherpesviruses, confirming that Helicase and primase (MDV017/UL5, MDV020/UL8
MDV is more closely related to alphaherpesviruses than gamma- and MDV066/UL52, H/P)
herpesviruses (Camp et al., 1991). The helicase/primase complex is a heterotrimer which contains
the products of the MDV017 or UL5, MDV020 or UL8, and
Proteins essential for viral DNA replication MDV066 or UL52 genes. UL5 protein contains conserved
ATP-binding and DNA helicase motifs, which are crucial for
Origin-binding protein (MDV021/UL9, OBP) DNA replication (Zhu and Weller, 1992a,b). The subcomplex
The MDV021 or UL9 gene encodes an 841 amino acid long formed by UL5 and UL52 shows DNA-dependent ATPase,
protein, with a 49% and 46% sequence identity to HSV-1 UL9 primase, and helicase activities, while UL8 interacts with other
and varicella-zoster virus (VZV) gene 51 product (VZV UL9), components of the replication machinery possibly coordinating
respectively. MDV OBP shares numerous structural motifs progression of the replication fork (Weller and Coen, 2012).
with HSV-1 and VZV UL9 proteins, including six conserved Recently it was shown that a single non-synonymous point
N-terminal helicase motifs, an N-terminal leucine zipper motif, a mutation (I682R) within the UL5 helicase-primase protein
C-terminal pseudo-leucine zipper sequence, and a putative helix- resulted in over 90% reduction in MDV virulence (Hildebrandt
turn-helix structure (Wu et al., 1996). MDV OBP recognizes the et al., 2015), suggesting that UL5 plays a critical role in MDV
sequence TTCGCACC, which is similar to that of HSV-1 OBP. DNA replication.