Avian Endogenous Retrovirus EAV-HP Shares Regions of Identity with Avian Leukosis Virus Subgroup J and the Avian Retrotransposon ART-CH

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Abstract
The existence of novel endogenous retrovirus elements in the chicken genome, designated EAV-HP, with close sequence identity to theenv gene of avian leukosis virus (ALV) subgroup J has been reported (L. M. Smith, A. A. Toye, K. Howes, N. Bumstead, L. N. Payne, and K. Venugopal, J. Gen. Virol. 80:261–268, 1999). To resolve the genome structure of these retroviral elements, we have determined the complete sequence of two proviral clones of EAV-HP from a line N chicken genomic DNA yeast artificial chromosome library and from a meat-type chicken line 21 lambda library. The EAV-HP sequences from the two lines were 98% identical and had a typical provirus structure. The two EAV-HP clones showed identical large deletions spanning part of the gag, the entirepol, and part of the env genes. Theenv region of the EAV-HP clones was 97% identical to theenv sequence of HPRS-103, the prototype subgroup J ALV. The 5′ region of EAV-HP comprising the R and U5 regions of the long terminal repeat (LTR), the untranslated leader, and the 5′ end of the putative gag region were 97% identical to the avian retrotransposon sequence, ART-CH. The remaining gagsequence shared less than 60% identity with other ALV sequences. The U3 region of the LTR was distinct from those of other retroviruses but contained some of the conserved motifs required for functioning as a promoter. To examine the ability of this endogenous retroviral LTR to function as a transcriptional promoter, the EAV-HP and HPRS-103 LTR U3 regions were compared in a luciferase reporter gene assay. The low luciferase activity detected with the EAV-HP LTR U3 constructs, at levels close to those observed for a control vector lacking the promoter or enhancer elements, suggested that these elements function as a weak promoter, possibly accounting for their low expression levels in chicken embryos.