RESISTANCE OF SPIROPLASMA-CITRI LINES TO THE VIRUS SVTS2 IS ASSOCIATED WITH INTEGRATION OF VIRAL-DNA SEQUENCES INTO HOST CHROMOSOMAL AND EXTRACHROMOSOMAL DNA

Spiroplasmavirus SVTS2, isolated from Spiroplasma melliferum TS2, prod uces plaques when inoculated onto lawns of Spiroplasma citri M200H, a derivative of the type strain Maroc R8A2. S. citri strains MR2 and MR3 , originally selected as colonies growing within plaques on a lawn of M200H inoculated with SVTS2, were resistant to SVTS2. Genomic DNA fing erprints and electrophoretic protein profiles of M200H, MR2, and MR3 w ere similar, but three proteins present in M200H were missing or signi ficantly reduced in both resistant lines, None of these three polypept ides reacted with antiserum against S. citri membrane proteins, indica ting that they probably are not surface-located virus receptors. Elect roporation with SVTS2 DNA produced 1.5 x 10(5) transfectants per mu g of DNA in M200H but none in MR2 or MR3, suggesting that resistance may result from inhibition of viral replication. The digestion patterns o f the extrachromosomal double-stranded (ds) DNA of these lines were si milar. Three TaqI fragments of MR2 extrachromosomal DNA that were not present in M200H extrachromosomal DNA hybridized strongly to an SVTS2 probe, and two of these fragments plus an additional one hybridized wi th the MR3 extrachromosomal DNA, indicating that a fragment of SVTS2 D NA was present in the extrachromosomal ds DNA of MR2 and MR3 but not o f M200H. When the restricted genomes of ail three lines were probed wi th SVTS2 DNA, strong hybridization to two EcoRI fragments of chromosom al MR2 and MR3 DNA but not M200H DNA indicated that SVTS2 DNA had inte grated into the genomes of MR2 and MR3 but not of M200H. When MR3 extr achromosomal ds DNA containing a 2.1-kb SVTS2 DNA fragment was transfe cted into M200H, the transformed spiroplasmas were resistant to SVTS2. These results suggest that SVTS2 DNA fragments, possibly integrated i nto the chromosomal or extrachromosomal DNA of a previously susceptibl e spiroplasma, may function as viral incompatibility elements, providi ng resistance to superinfection by SVTS2.