Several fowlpox virus (FPV) DNA fragments were selected by differential hybridization using cDNA of transcripts that were strongly transcribed early and/or late after infection of QT-35 cells. The RI L fragment contained three strongly transcribed FPV genes: L1L, a late 1452 bp partial (amino end) ORF; L2R, an early/late 522 bp ORF; and L3R, a late 948 bp ORF. The protein products of L1L, L2R and L3R shared homology with the products of vaccinia virus (VV) genes H4L (RAP94), H5R (Ag35) and H6R (topoisomerase), respectively, suggesting a conservation of gene structure and order between VV and FPV. The 5′ upstream non-coding sequences of L1L and L3R were A + T rich and the sequence 5′ TAAATG 3′ overlapped the predicted translation start codon. Primer extension analysis of the L2R transcript mapped the transcriptional start sites of early and late mRNAs 14 nt downstream of a VV early promoter-like critical region sequence, AAAATTGAAAAAAAAA. A VV-like TAAAT late transcriptional element was present 20 nt upstream of the L2R ATG translational start codon. A plasmid with the putative early L2R promoter cloned upstream of the Newcastle disease virus haemagglutinin-neuraminidase (HN) cDNA as a reporter gene was at least 6-fold more effective in generating HN mRNA than plasmids containing the P7.5 or P11 VV promoters in transient expression assays in FPV-infected CEF cells treated with cytosine arabinoside. The L2R promoter was also able to express an amount of HN mRNA equal to that expressed by the VV promoters late in infection.


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