The function of the a-rich region of the alphasatellite associated with the cotton leaf curl disease in Pakistan

A novel type of circular single stranded satellite-like DNA, known as alphasatellite (formerly known as DNA 1, was recently characterized and demonstrated to be associated with the monopartite Begomoviruses. Alphasatellite components are satellite like single stranded DNA (ssDNA) molecules associated with Begomoviruses (Geminiviridae) that require the betasatellite molecule to induce authentic disease symptoms in some hosts. Betasatellite is essential for induction of characteristic symptoms in plants. The function of alphasatellite in Begomovirus betasatellite infections remains unclear. It has been suggested that alphasatellite components may act to down regulate the virus infection by competing for cellular resources. Interestingly, they are closely related to the helper dependent Rep-encoding components of nanoviruses (a second family of single stranded, plant infecting DNA viruses), from which they are presumed to have been evolved. Alphasatellite molecules have two major sequence features. Firstly the component encodes a replication-associated protein (Rep), which is required to initiate the rolling circle replication. Consequently alphasatellite components are capable of self replication in host cells but require the helper Begomovirus to spread both within and between host plants. The second feature is a region of sequence rich in adenine (a-rich). To investigate the function of the a-rich sequence, this was deleted from the CLCuD alphasatellite by PCR mediated amplification. The a-rich deleted mutant of the alphasatellite remained capable of replication and systemic infection in plants, in the presence of a helper begomovirus. This indicates that the a-rich region is not required for replication or maintenance in plants.