THE 131-AMINO-ACID REPEAT REGION OF THE ESSENTIAL 39-KILODALTON CORE PROTEIN OF FOWLPOX VIRUS FP9, EQUIVALENT TO VACCINIA VIRUS A4L PROTEIN, IS NONESSENTIAL AND HIGHLY IMMUNOGENIC

The immunodominant, 39,000-molecular weight core protein (39K protein) of fowlpox virus (FP9 strain), equivalent to the vaccinia virus A4L g ene product, contains highly charged domains at each end of the protei n and multiple copies of a 12-amino-acid serine-rich repeat sequence i n the middle of the protein. Similar repeats were also detected in oth er fowlpox virus strains, suggesting that they might confer a selectiv e advantage to the virus. The molloscum contagiosum virus homolog (MC1 07L) also contains repeats, unlike the vaccinia virus protein. The num ber of repeats in the fowlpox virus protein does not seem to be crucia l, since some strains have a different number of repeats, as shown by the difference in the size of the protein in these strains. The repeat region could be deleted, indicating that it is not essential for repl ication in vitro. It was not possible to delete the entire 39K protein , indicating that it was essential (transcriptional control signals fo r the flanking genes were left intact). The repeat region is partly re sponsible for the immunodominance of the protein, but the C-terminal p art of the protein also contains highly antigenic linear epitopes. A r ole for the 39K protein in immune system modulation is discussed.