Transcriptional Regulation of Geminiviruses
The Geminiviridae are a family of plant infecting viruses, with a single stranded DNA genome, that are transmitted by insects. Geminivirus diseases are responsible for billions of dollars in annual losses of agricultural crops worldwide; therefore, development of disease resistance strategies is crucial. A single mRNA (AL1629) transcribed by Tomato golden mosaic virus (TGMV) directs expression of two important viral genes, AL2 and AL3. The AL2 protein suppresses host defenses and regulates the promoter for the coat protein gene, while the AL3 gene encodes a replication enhancement protein. Spinach curly top virus (SCTV) transcribes a similar mRNA (C2045) encoding the positional homologs C2 and C3 proteins.
My analysis demonstrates that the promoter sequences for both AL1629 and C2045 are conserved and active in the context of transient expression assays. For TGMV, binding of AL1 to the cognate binding site in the intergenic region resulting in prevention of initiation of the AL62 transcript has a dramatic effect on AL1629 promoter activation. Although the effect of AL1 was increased with replicon release, AL1 also seems to have a role independent of replication. Down-regulation of MET1 and CMT3 by AL1 may alter the chromatin from an inactive to active state as I do show a difference in DNA cytosine methylation and histone modifications for active and inactive AL1629 promoters. For SCTV, the result of C1 binding is not as significant. Thus, the TGMV and SCTV promoters ( AL1629 and C2045) seem to be conserved though may rely on different mechanisms for activation.