FIG. 1.

Left-terminal genome variation among strains of HVS and HVA. The variable regions of HVS subgroups A and C encode the proteins StpA, StpC, and Tip, respectively, which are necessary for the transforming phenotype of the virus. Two open reading frames (ORF1 and ORF2) were identified in the corresponding genomic region of HVA strain 73. This region was transcribed into an mRNA of about 1 kb. The transcript was found to be spliced and to code for a single protein, Tio, that has homology to Tip and StpC. The splice occurs within the StpC-homologous portion of the Tio mRNA. Two of the small U RNAs of HVS (HSUR and HAUR) and reading frame 3 (HVS and HVA 03) are conserved. The other HSUR copies and the reading frame for dihydrofolate reductase (DHFR) found in HVS are not present in the genome of HVA.