Mutational analysis of the putative nicking motif in the replication-associated protein (AC1) of bean golden mosaic geminivirus.

Geminiviruses are circular single-stranded DNA viruses that replicate by a rolling circle mechanism involving the viral-encoded AC1 protein. DNA nicking is necessary both for initiating replication of the covalently closed double-stranded DNA templates and for releasing unit-length monomers. The effects of mutations in a putative nicking motif (K101 A Y I D K106; E. V. Koonin and T. V. Ilyina, J. Gen. Virol. 73:2763-2766, 1992) of the AC1-derived protein for bean golden mosaic geminivirus isolate GA (BGMV-GA) were studied. The amino acids equivalent to Y103 and K106 of BGMV-GA are invariant in all whitefly-transmitted geminiviruses. Phaseolus vulgaris plant infectivity assays showed that the mutants K101-->H, K101-->A, and D105-->T produced symptoms, but mutants Y103-->A, Y103-->F, K106-->R, and K106-->H did not. A mutant with a stop codon in the N terminus of the AC4 open reading frame (ORF) produced the same symptoms as the wild-type BGMV-GA. Only those that were infectious replicated in NT-1 tobacco suspension cells. These results indicate that the Y103 and K106 residues are essential for replication, and that this putative DNA-nicking motif of the AC1 ORF may be functional in the rolling circle mechanism of replication for geminiviruses. The potential role of these mutants in the design of antiviral strategies is discussed.

Mutational analysis of a putative NTP-binding domain in the replication-associated protein (AC1) of bean golden mosaic geminivirus.

Bean golden mosaic virus (BGMV) is a whitefly-transmitted, ssDNA geminivirus with a bipartite genome. AC1 is the only ORF required for geminiviral replication. A putative NTP-binding motif, EGX4GKTX32DD, was present in the derived amino acid sequence of the replication-associated protein from the AC1 ORF for 13 geminiviruses including BGMV-GA (Guatemalan isolate, amino acids 221-263). We analyzed the phenotypes of mutations within this domain using a rapid and sensitive PCR-based assay for geminiviral replication developed for these studies. Replication in tobacco cells (NT-1 suspension cells) and infection of beans were abolished when codons were changed from K228 to H or D262 to R within the putative NTP-binding site. A temperature-sensitive replication phenotype was conferred by changing E221 to R within the putative NTP-binding domain. Replication was unaffected by changing a nonconserved codon near the putative NTP-binding domain from 1190 to R. Our results demonstrate that the putative NTP-binding domain is required for geminiviral replication. The role of NTP hydrolysis and the possible value of these mutants in a trans-dominant interference scheme for virus-derived resistance are discussed.