We have used "gene walking" with synthetic oligonucleotides and M13 dideoxynucleotide sequencing techniques to obtain the complete coding and flanking sequences of the gene encoding a major immediate-early RNA (molecular weight, 169,000) of frog virus 3. R-loop mapping of the cloned XbaI K fragment of frog virus 3 DNA with immediate-early RNA from infected cells showed that an RNA of approximately 500 to 600 nucleotides (the right size to code for the immediate-early viral 18-kilodalton protein of unknown function) hybridized to a region within 100 base pairs of one end of the XbaI K fragment; no evidence for splicing was observed in the electron microscope or by single-strand nuclease analysis. Further restriction mapping narrowed the location of the gene to the XbaI end of a 2-kilobase-pair XbaI-Bg/II fragment, which was bidirectionally subcloned into the bacteriophage pair mp10 and mp11 for sequencing. Mung bean nuclease mapping was used to identify both the 5' and the 3' ends of the mRNA. The 5' end mapped within an AT-rich region 19 base pairs upstream from two in-phase AUG start codons that were immediately followed by an open reading frame of 157 amino acids. Another AT-rich sequence was found at -29 base pairs from the 5' end of the mRNA start site; this sequence may function as a TATA box. The 3' end of the message displayed considerable microheterogeneity, but clearly terminated within a third AT-rich region 50 to 60 base pairs from the translation stop codon. The eucaryotic polyadenylic acid addition signal (AATAAA) was not present, a finding to be expected since frog virus 3 mRNA is not polyadenylated. Both the single-stranded mp10 clone of the XbaI-Bg/II fragment and a 15-base oligonucleotide complementary to the region flanking the two AUG translation start codons inhibited translation of the immediate-early 18-kilodalton protein in vitro, confirming the identity of the sequenced gene. As the regulatory sequences of this gene did not resemble those of known eucaryotic genes or of the cytoplasmic vaccinia virus, we conclude that frog virus 3 has evolved unique signals for the initiation and termination of transcription
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