【文章內(nèi)容簡介】 phor or Calcofluor, indicating a defect in the production of cellulose fibrils. Receptors are involved in initial binding ? Plant vitronectin(玻連蛋白 )like protein (PVN, 55kDa) was found on the surface of plant cell. This protein is probably involved in initial bacteria/plant cell binding. ? PVN is only immunologically related to animal vitronectin. ? Animal vitronectin is an important ponent of the extracellular matrix and is also an receptor for several bacterial strains. Receptors are involved in initial binding ? Aside from PVN, rhicadhesinbinding protein was found in pea roots. ? Also, rat1(arabinogalactan protein。 AGP) and rat2(potential cellwall protein) are involved. Plant signals ? Wounded plants secrete sap with acidic pH ( to ) and a high content of various phenolic pounds (lignin, flavonoid precursors) serving as chemical attractants to agrobacteria and stimulants for virgene expression. ? Among these phenolic pounds, acetosyringone (AS) is the most effective. Plant signals ? Sugars like glucose and galactose also stimulate vir gene expression when AS is limited or absent. These sugars are probably acting through the chvE gene to activate vir genes. ? Low opine levels further enhance vir gene expression in the presence of AS. Plant signals ? These pounds stimulate the autophosphorylation of a transmembranereceptor kinase VirA at its His474. ? It in turn transfers its phosphate group to the Asp52 of the cytoplasmic VirG protein. Plant signals ? VirG then binds to the vir box enhancer elements in the promoters of the virA, virB, virC, virD, virEand virG operons, upregulating transcription. ? Sugars interact with ChvE (glucose/galactose binding protein) which interacts with VirA through its periplasmic domain. Structure of the TDNA ? The existence and orientation of right border is absolutely required for Agrobacterium pathogenicity but not the left border. ? Transfer of the TDNA is polar from right to left. Structure of the TDNA ? Although right border and left border are required to delimit the transferred segments, the TDNA content itself has no effect on the efficiency of transfer. ? Therefore, researchers replace most of the TDNA with DNA of interest, making Agrobacterium a vector for geic transformation of plants. Production of Tstrand ? Every induced Agrobacterium cell produces one Tstrand. ? VirD1 and VirD2 are involved in the initial Tstrand processing, acting as siteand strandspecific endonucleases. Production of Tstrand ? After cleavage, VirD2 covalently attaches to the 5’ end of the Tstrand at the right border nick and to the 5’end of the remaining bottom strand of the Ti plasmid at the left border nick by its tyrosine 29. Production of Tstrand ? VirC1 enhances Tstrand production by binding to overdrive. ? Overdrive is a cisactive 24base pair sequence adjacent to the right border of the TDNA. It stimulates tumor formation by increasing the level of TDNA processing. Formation of the Tplex ? The Tplex is posed of at least three ponents: one Tstrand DNA molecule, one VirD2 protein, and around 600 VirE2 proteins. Formation of the Tplex ? Whether VirE2 associates with Tstrand before or after the intercellular transport is not clear. Formation of the Tplex ? If VirE2 associates with the Tstrand after intercellular transport, VirE1 is probably involved in preventing VirE2Tstrand binding. ? Judging from the size of the mature Tplex (13nm in diameter) and the inner dimension of Tpilus (10nm width), the Tstrand is probably associated with VirE2 after intercellular transport. Intercellular transport ? Transport of the Tplex into the host cell most likely occurs through a type IV secretion system. ? In Agrobacterium, the type IV transporter (called Tpilus) prises proteins encoded by virD4 and by the 11 open reading frames of the virB operon. Intercellular transport ? Intercellular transport of TDNA is probably energy dependent, requiring ATPase activities from VirB4 and VirB11. ? Physical contact between Agrobacterium and the plant cell is required to initiate Tplex export. Without recipient plant cells, Tstrands accumulate when vir genes are induced. Nuclear Import ? Because the large size of Tplex (50,000 kD, ~13nm in diameter), the nuclear import of Tplex requires active nuclear import. ? The Tplex nuclear import is presumably mediated by the Tplex proteins, VirD2 and VirE2. Both of them have nuclearlocalizing activities. Nuclear Import ? VirD2 is imported into the cell nucleus by a mechanism conserved between animal, yeast and plant cells (bipartite consensus motif). ? VirE2 has a plantspecific nuclear localization mechanism. It does not localize to the nucleus of yeast or animal cells. Nuclear Import ? In host plant cells VirD2 and VirE2 likely cooperate with cellu