Gene Arrangement within the Unique Long Genome Region of Infectious Laryngotracheitis Virus Is Distinct from That of Other Alphaherpesviruses

The genome of the avian alphaherpesvirus infectious laryngotracheitis virus (ILTV) comprises ca. 155 kbp of which ca. one-third have been sequenced so far. To gain additional sequence information we analyzed two stretches of 15.5 and 1.9 kbp of the ILTV unique long (U_L) genome region. The larger fragment contains homologs of the herpes simplex virus (HSV) UL23 (thymidine kinase) and UL22 (glycoprotein H) genes followed by five open reading frames (ORF) encoding putative proteins of 334 to 410 amino acids which exhibit no homology to any known herpesvirus protein. RNA analyses showed that these unique ILTV genes are indeed expressed. An origin of replication separates this cluster of unique genes from a conserved gene cluster consisting of the UL45, UL46, UL48, UL49, UL49.5, and UL50 homologs. The absence of UL47 from this position coincides with the localization of a UL47-homologous ORF within the unique short (U_S) region of the ILTV genome (M. Wild, S. Cook, and M. Cochran, Virus Genes 12:107–116, 1996). Within the second analyzed region the ILTV UL21 homolog was found adjacent to the UL44 gene. We thus identified five novel herpesvirus genes in ILTV and present evidence for a large internal inversion in the ILTV U_L region, in contrast to the collinear genomes of other alphaherpesviruses. Interestingly, a similar inversion is also present in the porcine alphaherpesvirus pseudorabies virus.