Mutational Analysis of the Virion-sense Genes of Maize Streak Virus

Abstract

SUMMARYInsertion and deletion mutagenesis of the two virion-sense genes, V1 and V2, of maize streak virus (MSV) prevents symptomatic infections following Agrobacteriummediated 'agroinoculation' of maize seedlings. These genes code for an Mr 10900 protein and for coat protein, respectively. Mutants containing insertions or deletions in the coat protein gene, V2, were able to replicate to low levels, producing dsDNA although virion ssDNA was not detected and symptoms were not observed. Hence, unlike the bipartite geminiviruses, MSV requires coat protein to produce symptomatic systemic infection. Mutations in gene V1 which considerably shortened the Mr 10900 protein (V1 gen¢) resulted either in low levels of replication, in which all the DNA forms associated with a wild-type infection were produced, or in no infection, in which case coat protein production may also have been affected. A V1 mutant generated in vivo with 11 of the 14 N-terminal amino acids altered, was viable and produced symptoms typical of a wild-type infection. Infectivity, assessed by replication and symptom expression, was restored by co-inoculating constructs containing single mutations in different open reading frames, thus rescue can occur by transcomplementation of gene products. The experiments showed that the mutations did not affect the nucleotide sequence requirements for replication and that in all cases intermolecular recombination eventually resulted in dominant wild-type virus.

Mutational Analysis of Effectors Encoded by Monopartite Begomoviruses and Their Satellites

“…Mutational analysis of few monopartite begomoviruses like TYLCV define similarities and differences between this single component geminivirus and bipartite geminiviruses in functions essential for systemic spread and infectivity [103]. The CP appeared to be required for systemic movement of TYLCV in N. benthamiana and tomato, consistent with those of mutation analyses obtained with other monopartite geminiviruses such as MSV, BCTV, and TLCV [5,12,69,61] have shown that Tomato yellow leaf curl virus-Sardinia (TYLCV-Sar) C2 can form stable complexes with ssDNA (and less preferably with dsDNA) and that the binding is sequence nonspecific. AC2 of TYLCV have also been involved in the activation of other viral genes and was considered as a transcriptional activator [91].…”

“…Besides the encapsidation function, CP is also required for transmission of the virus between the plants. The CP of the monopartite begomoviruses facilitates the transfer of infecting viral DNA into the host cell nucleus and is essential for systemic virus movement [5,46,50,104]. The CP also determines the vector specificity [10,32,33] and protects the viral ssDNA from degradation during transmission by the insect vector [3], or mechanical inoculation [24].…”

Genome typing of southern African subgroup 1 geminiviruses

“…Sequences were obtained using pUC universal (forward) sequencing primer. Boulton et al (1989). different viruses (and their strains) are ubiquitous in Africa.…”

Efficient whitefly transmission of African cassava mosaic geminivirus requires sequences from both genomic components.

“…For mutations within the CP, which abolish transmission, a selective advantage is more difficult to imagine. Possibly the CP of bipartite WTGs play a part in cell-to-cell movement (Jeffrey et al, 1996), as is the case for monopartite geminiviruses (Briddon et al, 1989 ;Boulton et al, 1989). Certainly these findings highlight the pitfalls of using ' laboratory-adapted ' virus strains for cloning and analysis, as these may not be a true representation of the field isolate.…”