Figure 1.
Schematic of ‘free’ Marek's disease virus: linear and episomal genomes. Diagrams of free (non-integrated) viral genome in the linear and circularized (episomal) forms are depicted. The MDV genome consists of double-stranded DNA (all serotypes) and is approximately 180kb (serotype 1). The MDV genome structure is divided into long and short sections, similar to other alpha-herpesviruses. Both the long and short (denoted by L and S subscripts, respectively) sections consist of: unique (U) sequence regions, blocks of sequences that are corresponding inverted repeats of one another (terminal (TR) and internal (IR)), telomeric DNA sequence ((TTAGGG)n) blocks, and alpha-like sequence which includes telomeric DNA sequence (Kishi et al., 1988; Lee et al., 2000a; Tulman et al., 2000). The colored blocks indicate the locations of the (TTAGGG)n telomeric sequence repeats (green) and the blocks of inverted sequences within the long (dark blue, TRL and light blue, IRL) and short (dark purple, IRS and light purple, TRS) sections. The straight lines within the long and short sections represent the unique sequence regions (UL and US). The (TTAGGG)n sequences, viral telomerase RNA (vTR) gene and Meq gene locations are indicated by arrows (see orange block notation). Notably, upon circularization of the genome, the two blocks of telomeric sequence repeats are brought together. Table 1 describes key differences and similarities between the MDV serotypes with regard to presence/absence and/or functionality of viral telomeric DNA sequence and the vTR and Meq genes.