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==T-DNA region==
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 condensation between amino acids and sugars, are synthesized and excreted by the crown gall cells and consumed by A. tumefaciens as carbon and nitrogen sources. Outside the T-DNA, are located the genes for the opine catabolism, the genes involved in the process of T-DNA transfer from the bacterium to the plant cell and the genes involved in bacterium-bacterium plasmid conjugative transfer. (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 that are 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). Different chromosomal-determined genetic elements have shown their functional role in the attachment of ''A. tumefaciens'' to the plant cell and bacterial colonization: the loci chvA and chvB, 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) the cell locus, responsible for the synthesis of cellulose fibrils (Matthysse 1983); the {{not a typo|pscA (exoC)}} locus playing its role in the synthesis of both cyclic glucan and acid succinoglycan (Cangelosi et at. 1987, 1991) and the att locus, which is involved in the cell surface proteins (Matthysse, 1987).
==References==
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