The primary function of the virD operon is to encode proteins required to facilitate endonucleolytic cleavage of T-DNA. The operon is divergently transcribed with the virC operon and is composed of 4 genes - virD1 (which encodes a topoimerase), virD2 (which encodes an endonuclease), virD3 (function is not well characterized) and virD4 (encodes a protein that functions as a coupling factor, linking the T-DNA/VirD2 complex to the Type IV secretion system (T4SS) encoded by other vir operons (e.g., virB), facilitating the transfer of T-DNA into the plant cell). VirD1 and virD2 facilitate T-DNA transfer to plant cells by nicking the 25-bp right- and left-border sequence of the T-DNA. Following the nicking, virD2 protein covalently binds to the 5’ border of the T-DNA and helps direct the TDNA into the plant cell nucleus. Expression is controlled by virA which encodes for transmembrane histidine protein kinase, virG which encodes a cytoplasmic response regulator and the periplasmic sugar-binding protein ChvE. AS binds to transmembrane histidine protein kinase virA, resulting in phosphorylation of virG which then binds to the vir box (10 to 12bp sequences) in the promoter region and initiates transcription. The operon transcription is repressed by a chromosomal gene known as Ros which encodes for a transcriptional regulator and competes with virG thus countering infection.

Virulence (virD operon activation) References