Multiple alternative splicing to exons II and III of viral interleukin-8 (vIL-8) in the Marek's disease virus genome: The importance of vIL-8 exon I

The Marek's disease virus (MDV) Eco Q (Meq) and the interleukin-8 (IL-8) MDV homologue (vIL-8) genes, and the open reading frames RLORF5a and RLORF4 are encoded within the repeat long (IR_L and TR_L) regions of the MDV genome. The recent cloning and characterization of RLORF4 led to the identification of a RLORF4/vIL-8 splice variant using 3′ rapid amplification of cDNA ends (RACE). Further characterization of 3′RACE products amplified with primers located within the Meq, RLORF5a, or RLORF4 genes showed the presence of many splice variants. Two novel Meq splice variants were detected, in addition to splice variants encoding portions of RLORF5a and RLORF4 combined with exons II and III of vIL-8 (RLORF5a/vIL-8 and RLORF4/vIL-8, respectively). Analysis of expression in MDV-infected chickens showed that the RLORF5a/vIL-8 and 3 of 4 RLORF4/vIL-8 transcripts were only expressed at 4 days post-infection. Since a number of transcripts encoded vIL-8 exons II and III, this suggested that exon I may be non-essential for vIL-8 function(s). Virus reconstituted from the oncogenic pRB-1B bacterial artificial chromosome with vIL-8 exon I deleted showed decreased early replication and reduced incidence of tumor development, similar to deletion mutants lacking the complete vIL-8 gene.