NUCLEOTIDE AND AMINO-ACID VARIATION IN THE COAT PROTEIN-CODING REGIONOF TURNIP MOSAIC-VIRUS ISOLATES AND POSSIBLE INVOLVEMENT IN THE INTERACTION WITH THE BRASSICA RESISTANCE GENE TURBO1

The nucleotide and predicted amino acid sequences of the coat protein coding region of four isolates (UK 1, CZE 1, CDN 1 and GK 1) and a put ative mutant of one of these isolates (UK 1M) of turnip mosaic virus ( TuMV) were determined. Comparisons between these and those of previous ly sequenced isolates showed homologies of 88.6-99.9% in nucleotides a nd 93.4-100% in amino acid sequences. Nucleotide substitutions were re vealed at 180 different positions that give rise to 30 predicted amino acid changes. The coal protein coding regions consisted of 864 nucleo tides, with the exception of GK 1 which had 861, having a deletion in amino acid position 44. This is the first recorded deletion in the coa t protein coding region of TuMV. Alignment of the predicted amino acid sequences of UK 1, CZE 1, CDN 1 and GK 1 isolates with the previously sequenced CDN 2, CHN,JPN I,JPN 2,JPN (31), L12396 and KOR isolates re vealed three distinct groups which, with a few exceptions, were correl ated with the geographical and/or host plant origin of isolates. The G K 1 isolate showed the lowest nucleotide (88.6-89.9%) and amino acid ( 93.4-96.2%) homologies with the other isolates, whereas the UK 1 isola te showed the highest nucleotide homology (89.7-99.9%) and CDN 1 showe d the highest amino acid homology (95.9-99%). The coat protein coding region of the putative mutant isolate (UK 1M) had a single nucleotide difference (G to A) from UK 1 at position 9 which did not give rise to any change in the predicted amino acid sequence. UR 1M is able to inf ect Brassica napus plants possessing a mapped resistance gene (TuRB01) that is effective against UK 1. The phenotypes of most of the sequenc ed isolates on plants containing TuRB01 are known, and these are consi stent with the hypothesis that the single nucleotide substitution at p osition 9 of the coat protein coding region conditions the interaction with this gene. All isolates tested that had the residue G(9) gave no detectable replication in plants possessing TuRB01, whereas isolates that had the residue A(9) were either partially virulent on plants car rying TuRB01 causing local infection only, or were fully virulent resu lting in full systemic infection. This hypothesis is unprecedented in plant virus interactions and fur;her experimentation utilizing full-le ngth infectious clones of TuMV will be necessary to test its validity. (C) 1997 Academic Press Limited.