Generation of packaging-defective DNA molecules of equine adenovirus

Equine adenovirus (EAd) DNA prepared from infected bovine kidney (MDBK) cells contained additional sequences of about 100 to 700 bp at the left-hand end of the genome. These aberrant viral genomes were produced even after the first passage of the wild type EAd in MDBK cells and their relative amounts did not change significantly during serial passage. The left terminal fragments of two defective viral DNAs were cloned into the plasmid vector pBR322 and the nucleotide sequences of their terminal regions were analyzed. The data indicate that one viral DNA contained a duplication of the inverted terminal repetition (ITR) and the other contained 270 bp of additional sequences derived from the right-terminal region of EAd genome added to the left-terminal, ITR. While the former DNA was packaged into virions, the latter was not, presumable due to the alteration of the distance from the left terminus to the putative DNA packaging signal, reported to be located between 290 and 390 bp (Hammarskjold and Winberg, 1980). The possible mechanism for the generation of these defective DNAs is discussed.