RNA STRUCTURE AND HETEROLOGOUS RECOMBINATION IN THE DOUBLE-STRANDED-RNA BACTERIOPHAGE PHI-6

Bacteriophage phi6 has a genome of three segments of double-stranded R NA, designated L, M, and S. A 1.2-kbp kanamycin resistance gene was in serted into segment M but was shown to be genetically unstable because of a high recombination rate between segment M and the 3' ends of seg ments S and L. The high rate of recombination is due to complementary homopolymer tracts bounding the kan gene. Removal of one arm of this p otential hairpin stabilizes the insertion. The insertion of a 241- or 427-bp lacZ' gene into segment M leads to a stable Lac+ phage. The ins ertion of the same genes bounded by complementary homopolymer arms lea ds to recombinational instability. A stable derivative of this phage w as shown to have lost one of the homopolymer arms. Several other condi tions foster recombination. The truncation of a genomic segment at the 3' end prevents replication, but such a damaged molecule can be rescu ed by recombination. Similarly, insertion of the entire 3-kb lacZ gene prevents normal formation of virus, but the viral genes can be rescue d by recombination. It appears that conditions leading to the retardat ion or absence of replication of a particular genomic segment facilita te recombinational rescue.