A Single Complementary-Sense Transcript of a Geminiviral DNA β Satellite Is Determinant of Pathogenicity

Abstract

Small circular single-stranded DNA satellites, termed DNAβ, have recently been found associated with some geminivirus infections. The DNA β associated with Cotton leaf curl virus is responsible for symptom expression of a devastating disease in Pakistan. Mutagenesis of DNA β revealed that the complementary-sense open reading frame (ORF) βC1 is required for inducing disease symptoms in Nicotiana tabacum. An ORF present on the virion-sense strand βV1 appeared to have no role in pathogenesis. Tobacco plants transformed with a βC1 ORF under the control of the Cauliflower mosaic virus 35S promoter or with a dimeric DNA β exhibited severe disease-like phenotypes, while plants transformed with a mutated version of βC1 appeared normal. Northern blot analysis of RNA from the transgenic plants, using strand-specific probes, identified a single complementary-sense transcript. The transcript carries the full βC1 ORF encoding a 118-amino acid product. It maps to the DNA β at nucleotide position 186 to 563 and contains a polyadenylation signal 18 nt upstream of the stop codon. A TATA box is located 43 nt upstream of the start codon. Our results indicate that βC1 protein is responsible for DNA β-induced disease symptoms.