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T-DNA contains two types of genes: the oncogenic genes, encoding for enzymes involved in the synthesis of auxins and cytokinins and responsible for tumor formation, and the genes encoding for the synthesis of opines. These compounds, produced by the condensation between amino acids and sugars, are synthesized and excreted by the crown gall cells, and they are consumed by A. tumefaciens as carbon and nitrogen sources.
Outside the T-DNA, are located theThe genes involved in opine catabolism, transferring T-DNA transfer from the bacterium to the plant cell, and bacterium-bacterium plasmid conjugative transfer are located outside the T-DNA (Hooykaas and Schilperoort, 1992; Zupan and Zambrysky, 1995). The T-DNA fragment is flanked by 25-bp direct repeats, which act as a cis-element signal for the transfer apparatus. The process of T-DNA transfer is mediated by the cooperative action of proteins encoded by genes determined in the Ti plasmid virulence region (vir genes) and in the bacterial chromosome. The Ti plasmid also contains the genes for opine catabolism produced by the crown gall cells and regions for conjugative transfer and for its own integrity and stability. The 30 kb virulence (vir) region is a regulon organized in six operons essential for the T-DNA transfer (virA, virB, virD, and virG) or for the increasing of transfer efficiency (virC and virE) (Hooykaas and Schilperoort, 1992; Zupan and Zambryski, 1995, Jeon et al., 1998). DifferentSeveral chromosomal-determined genetic elements have shown their functional role in the attachment of ''A. tumefaciens'' to the plant cell and bacterial colonization:. theThe loci chvA and chvB, are involved in the synthesis and excretion of the b -1,2 glucan (Cangelosi et al., 1989), the {{not a typo|chvE}} required for the sugar enhancement of vir genes induction and bacterial chemotaxis (Ankenbauer et al., 1990, Cangelosi et al., 1990, 1991). theThe cell locus, is responsible for the synthesis of cellulose fibrils (Matthysse 1983);. theThe {{not a typo|pscA (exoC)}} locus playingis its roleinvolved in the synthesis of both cyclic glucan and acid succinoglycan (Cangelosi et at. 1987, 1991). and theThe att locus, which is involved in the cell surface proteins (Matthysse, 1987).
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