The nucleotide sequence of the helper component protease (HC-Pro) genes of three zucchini yellow mosaic virus (ZYMV) strains has been compared with that of a helper-deficient strain of ZYMV-HC. The comparisons revealed three unique deduced amino acid differences. Two of these mutations were located in regions which are conserved in other potyviruses. The role of these mutations in aphid transmissibility was examined by exchanging DNA fragments of part of the deficient HCPro gene with the respective section within the gene of the infectious full-length clone of the aphid-transmissible ZYMV. The first exchange included two of the three mutations, the first coding for a change from Asp to Gly (in a non-conserved region) and the second coding for a change from Arg to Ile [within the Phe-Arg-Asp-Lys (FRNK) conserved box]. This exchange resulted in a reduced transmission (20-6% for the mutated virus compared with 57-4% in the normal ZYMV when acquired from plants and 37.2 % compared with 83"1%, respectively, when acquired from membranes). The second exchange incorporated a single mutation [conferring a change from Thr to Ala within the Pro-Thr-Lys (PTK) conserved box]. This single mutation resulted in almost total loss of HC activity in aphid transmission both from plants and from membranes. The Lys residue in the conserved Lys-Ile-Thr-Cys (KITC) box, which is related to loss of HC activity in potato virus Y, tobacco vein mottling virus and in the Michigan strain of ZYMV, is unchanged in the helper-deficient ZYMV. It is therefore proposed that more than one site in HC-Pro may be functionally related to aphid transmissibility. The possible reasons for the role of these mutations in helper activity in aphid transmission of ZYMV are discussed.
An rDNA size class in the genome of the nematode Ascaris lumbricoides is described which is interrupted by a 4.5‐kb long intervening sequence located in the 26S coding region. This molecular form occurs in approximately 15 copies per haploid genome and amounts to approximately 5% of the total nuclear rDNA. Intervening sequences are present only in the 8.8‐kb rDNA, but not in the 8.4‐kb rDNA repeating units of A. lumbricoides. Cloning of the interrupted rDNA units revealed, in addition to the main 4.5‐kb insertion, shorter intervening sequences of 4‐kb and 119‐bp length. Both shorter rDNA forms are present in the single copy range of the haploid genome. Sequence analyses of the intervening sequence/rDNA junctions show an identical right‐hand junction for all of the three different rDNA forms. The two shorter intervening sequences are a coterminal subset of the right‐hand end of the main 4.5‐kb insertion, whereas all three insertions have a different left‐hand junction with the coding region of rDNA. Each intervening sequence is flanked by a short direct repeat of variable length, being only once present in the uninterrupted rDNA. The intervening sequences of A. lumbricoides show striking similarity to the organization of type I insertion family in dipteran flies, even though they are inserted at different positions in the 26S coding region. Additional rDNA intervening sequences may be present outside of the rDNA cluster, but in not more than 15‐20 homologous copies per haploid genome.
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