Many monopartite begomoviruses are associated with betasatellites, but only several promoters from which were isolated and studied. In this study, the βC1 promoter from Malvastrum yellow vein betasatellite (MYVB) was characterized and important sequence elements were identified to modulate promoter activity and replication of MYVB.
A 991 nucleotide (nt) fragment upstream of the translation start site of the βC1 open reading frame of MYVB and a series of deletions within this fragment were constructed and fused to the β-glucuronidase (GUS) and green fluorescent protein (GFP) reporter genes, respectively. Agrobacterium-mediated transient expression assays showed that the 991 nt fragment was functional and that a 28 nt region (between −390 nt and −418 nt), which includes a 5′UTR Py-rich stretch motif, was important for promoter activity. Replication assays using Nicotiana benthamiana leaf discs and whole plants showed that deletion of the 5′UTR Py-rich stretch impaired viral satellite replication in the presence of the helper virus. Transgenic assays demonstrated that the 991 nt fragment conferred a constitutive expression pattern in transgenic tobacco plants and that a 214 nt fragment at the 3'-end of this sequence was sufficient to drive this expression pattern.
Our results showed that the βC1 promoter of MYVB displayed a constitutive expression pattern and a 5′UTR Py-rich stretch motif regulated both βC1 promoter activity and MYVB replication.