A self-excising Cre recombinase allows efficient recombination of multiple ectopic heterospecific lox sites in transgenic tobacco

To study the impact of different DNA configurations on the stability of transgene expression, a variant of the cre gene was developed. This variant allows for the highly efficient in planta removal of its own loxP-flanked coding sequence as well as other DNAs flanked by ectopic heterospecific lox sites, either lox511 or lox2272 or both, in trans. The plant intron-containing cre gene, cre(INT), was configured in such a way that self-excision generated an intact hygromycin resistance selectable marker gene. In this combination, all selected transformants showed highly efficient excision. Plants obtained showed no indication of any chimerism, indicating a cell autonomous nature of the hygromycin selection during transformation and regeneration. The highly efficient concomitant removal of wildtype and heterospecific lox site-flanked DNA demonstrated that upon retransformation with the self-excising cre(INT), sufficient amounts of Cre enzyme were produced prior to its removal. Plants obtained with cre(INT) showed much less frequently the Cre-associated phenomenon of reduced fertility than plants obtained with a continuous presence of Cre recombinase. The cre(INT) system has therefore advantages over systems with a continuously present Cre. The cre(INT) system was successfully used for removal of two chromatin boundary elements from transgene cassettes in tobacco. Analysis of plants with and without boundary elements on the same chromosomal location will contribute to a better evaluation of the role of such elements in the regulation of transgene expression in plants.