ISOLATION AND MOLECULAR CHARACTERIZATION OF DTNP1, A MOBILE AND DEFECTIVE TRANSPOSABLE ELEMENT OF NICOTIANA-PLUMBGINIFOLIA

By Northern blot analysis of nitrate reductase-deficient mutants of Ni cotiana plumbaginifolia, we identified a mutant (mutant D65), obtained after gamma-ray irradiation of protoplasts, which contained an insert ion sequence in the nitrate reductase (NR) mRNA. This insertion sequen ce was localized by polymerase chain reaction (PCR) in the first exon of NR and was also shown to be present in the NR gene. The mutant gene contained a 565 bp insertion sequence that exhibits the sequence char acteristics of a transposable element, which was thus named dTnp1. The dTnp1 element has 14 bp terminal inverted repeats and is flanked by a n g-bp target site duplication generated upon transposition. These inv erted repeats have significant sequence homology with those of other t ransposable elements. Judging by its size and the absence of a long op en reading frame, dTnp1 appears to represent a defective, although mob ile, transposable element. The octamer motif TTTAGGCC was found severa l times in direct orientation near the 5' and 3' ends of dTnp1 togethe r with a perfect palindrome located after the 5' inverted repeat. Sout hern blot analysis using an internal probe of dTnp1 suggested that thi s element occurs as a single copy in the genome of N. plumbaginifolia. It is also present in N. tabacum, but absent in tomato or petunia. Th e dTnp1 element is therefore of potential use for gene tagging in Nico tiana species.