Mechanism of poly(A) synthesis by vaccinia virus.

Data are presented which indicate that vaccinia DNA does not contain poly(dT) sequences the size of poly(A) sequences (50 to 200 nucleotides in length) found in vaccinia RNA. A hybridization experiment and polyacrylamide gel electrophoresis and DEAE-Sephadex chromatography of pyrimidine tracts show that poly(dT) sequences can account for no more than 0.1% of vaccinia DNA. Ultraviolet irradiation (which causes thymine dimer formation) and phleomycin (which binds to thymidine) both inhibit RNA synthesis but not poly(A) synthesis by vaccinia cores. These data are consistent with a nontranscriptive mechanism for vaccinia poly(A) synthesis. Both trypsin and 50 C heat treatment inhibit RNA synthesis more than poly(A) synthesis by cores, suggesting that separate enzymes may be involved in these syntheses. When the rate of core RNA synthesis is reduced by lowering the UTP and GTP concentrations, the size of the poly(A) sequences increase. These and other data suggest that transcription is involved in the termination of poly(A) synthesis in cores. This might be due to the displacement of growing poly(A) chains by recently completed RNA 3' termini which have not yet acquired poly(A) sequences.