regulation of agrobacterium tumefaciens tcyt gene expression in leaves of transgenic potato solanum tuberosum l cv dsire is strongly influenced by plant culture conditions the promoter region of the agrobacterium tumefaciens tcyt gene was linked in a translational fusion to the coding dna of the reporter gene uida for betaglucuronidase or gus protein ec 32131 and to nos 3 flanking dna the chimaeric gene was introduced by agrobacterium transformation into potato solanum tuberosum l cv dsire in nine transgenic lines the average gus levels were highest in extracts from stems and roots of in vitro grown plants ca 11000 gus activity units per pmol mu per mg protein per min but lower in leaves of the in vitro grown plants ca 7000 units gus activity was intermediate in stems and roots of plants grown in soil as well as in in vitro crown galls ca 3000 units activity was low in tubers irrespective of whether these developed in vitro or in soil both ca 100 units and lowest of all in leaves of soilgrown plants ca 1015 units however in shoot cultures reestablished from soilgrown plants gus activity in the leaves increased to that determined in the original shoot cultures hence plant culture conditions strongly influenced the expression of the tcytuidanos gene in particular it was silenced in leaves of soilgrown plants the results are compared with previous analyses of the promoter region of the wildtype tcyt gene and with the growth properties of a large number of crown gall cell lines and crowngallderived plants including over forty s tuberosum cv dsire cell lines isolated in the present study that were transformed with the wildtype tcyt gene and six promotermutated derivatives a number of implications are discussed for crown gall formation and for control of expression of plant genes which contain activator or gbox type 5 expression control sequences