Structural organization of representative members of Beta vulgaris chromoviruses. Open arrows represent the long terminal repeats (LTRs). The primer binding site (PBS) and the polypurine tracts (PPTs) are indicated. Conserved domains are shown: gag and pol, with the catalytic domains of protease (PR), reverse transcriptase (RT), RNaseH (RH), and integrase (INT), containing the chromodomain (CD) at the C-terminus. Note that the ORF of the gag-pol polyprotein in Beetle 7 and Bingo 1 extends into the 3′ LTR. Zinc finger (ZF) motifs of the protease and integrase are indicated. The additional PPT of Bongo 3 is designated PPTa. Numbered dashes show the position of probes used for hybridization experiments. The corresponding primers used for DNA probe amplification are listed in Table 2.