A T-DNA GENE REQUIRED FOR AGROPINE BIOSYNTHESIS BY TRANSFORMED PLANTSIS FUNCTIONALLY AND EVOLUTIONARILY RELATED TO A TI PLASMID GENE REQUIRED FOR CATABOLISM OF AGROPINE BY AGROBACTERIUM STRAINS

The mechanisms that ensure that Ti plasmid T-DNA genes encoding protei ns involved in the biosynthesis of opines in crown gall tumors are alw ays matched by Ti plasmid genes conferring the ability to catabolize t hat set of opines on the inducing Agrobacterium strains are unknown. T he pathway for the biosynthesis of the opine agropine is thought to re quire an enzyme, mannopine cyclase, coded for by the ags gene located in the T-R region of octopine-type Ti plasmids. Extracts prepared from agropine-type tumors contained an activity that cyclized mannopine to agropine. Tumor cells containing a T region in which ags was mutated lacked this activity and did not contain agropine. Expression of ags f rom the lac promoter conferred mannopine-lactonizing activity on Esche richia coli. Agrobacterium tumefaciens strains harboring an octopine-t ype Ti plasmid exhibit a similar activity which is not coded for by ag s. Analysis of the DNA sequence of the gene encoding this activity, ca lled agcA, showed it to be about 60% identical to T-DNA ags genes. Rel atedness decreased abruptly in the 5' and 3' untranslated regions of t he genes. ags is preceded by a promoter that functions only in the pla nt. Expression analysis showed that agcA also is preceded by its own p romoter, which is active in the bacterium. Translation of agcA yielded a protein of about 45 kDa, consistent with the size predicted from th e DNA sequence. Antibodies raised against the agcA product cross-react ed with the anabolic enzyme. These results indicate that the agropine system arose by a duplication of a progenitor gene, one copy of which became associated with the T-DNA and the other copy of which remained associated with the bacterium.