Differences in intracellular DNA ligation after microinjection and transfection.

An uninterrupted avian sarcoma viral genome terminated by viral long terminal repeat sequences was cloned into a pBR322 plasmid. After introduction into a cultured avian cell, transcription of either the circular plasmid molecule or one linearized within the pBR322 sequences could initiate and terminate at long terminal repeat sequences, yielding full-sized viral RNA. A plasmid DNA molecule linearized by cleavage within the viral pol gene, on the other hand, would have to undergo ligation to yield full-sized viral RNA. Microinjection of each of these three types of DNA into the nuclei of quail cells promoted the release of similar virus titers, indicating that the plasmid DNA cleaved within the viral pol gene had been efficiently and accurately ligated. When plasmid DNA was transfected into quail cells, circular and pBR322-cleaved molecules directed the synthesis of similar virus titers, indicating that they were similarly taken up and utilized by the cells. Compared with these results, plasmid DNA cleaved within the pol gene was reduced in activity over 95% after transfection. This reduction did not result from inefficient ligation but from the generation of mutations (of limited size) during ligation of the transfected molecules. Mutations were not observed after microinjection even into the cytoplasm. Consistent with these findings, transfected DNA termini were found to be joined regardless of their structure, whereas ligation after microinjection required that single-stranded protruding DNA termini be complementary.