Extended sequence analysis of three Danish potato mop-top virus (PMTV) isolates.

The entire nucleotide sequence for the coding regions of a Danish PMTV isolate 54-15 was determined and compared to other known and sequenced isolates of PMTV. Many nucleotide and amino acid changes were found in parts of RNA coding for the triple gene block (TGB) proteins and in the part of the RNA coding for the read-through region of the coat protein (CP). These regions for two other isolates, the mild one 54-10 and the severe one 54-19, were sequenced. Only two amino acid changes were found to correlate with the subdivision of isolates according to symptom development into mild and severe subgroups. In addition, the phylogenetic tree was obtained suggesting the closest relationship between isolates 54-15 and 54-10. Although the sequence comparisons indicate a high genetic stability of PMTV populations, a surprising change was found in the newly sequenced isolates--the replacement of the AUG start codon of the fourth gene of the TGB encoding RNA, coding for a cystein-rich protein, by the less efficient GUG start codon.