LARGE DELETIONS IN ANTIGENIC VARIANTS OF THE CHLORELLA VIRUS PBCV-1

Four spontaneously derived, antigenic variants of chlorella virus PBCV -1 contained 27- to 37-kb deletions in the left end of the 330-kb geno me. Two of the mutants, which were serologically identical, had deleti ons that began from map position 4.9 or 16 and ended at position 42.2 kb. In total, the two deleted regions encoded 28 putative functional o pen reading frames (ORFs); these deletions probably arose from homolog ous recombination. The other two mutants, which were serologically ide ntical but distinct from the first two mutants, lacked the entire left terminal 37 kb of the PBCV-1 genome, including an identical 2.2-kb in verted terminal repeat region present at both ends of the wild-type ge nome. The deleted left end region was replaced by the transposition of an inverted 7.7- or 18.5-kb copy of the right end of the PBCV-1 genom e. The region deleted in these two viruses encoded 26 single-copy ORFs , of which 23 were common to those deleted in the first two mutant vir uses. The junctions of the deletions/transpositions probably arose fro m nonhomologous recombination. Taken together, the results indicate th at 40.1 kb of single-copy DNA encoding 31 ORFs at the left end of the genome are unnecessary for PBCV-1 replication in Chlorella strain NC64 A in the laboratory. The results also indicate that the size of the in verted terminal repeat region in this virus can be highly variable and that the PBCV-1 DNA packaging process tolerates large changes in geno me size. (C) 1995 Academic Press, Inc.