MOLECULAR INSERTION-SITE SELECTIVITY OF DS IN TOMATO

The authors analyzed, by pulsed field gel electrophoresis (PFGE), the physical distribution of 29 independent genetically linked Ds transpos itions from a T-DNA donor locus on tomato chromosome 6. While no prefe rences of transpositions with respect to direction or orientation were found, 28 transpositions occurred over short distances (<250kb) and w ere found in distinct clusters of 10-50kb separated by empty regions o f 60-100kb. The strongest cluster received 7 hits in a similar to 10kb area at 55kb from the T-DNA, representing a hot spot in 220kb of surr ounding refractory DNA. A Cre-lox mediated deletion analysis of refrac tory areas was performed, but these remained largely inconclusive due to an impossibility to germinally transmit the deletions. The existenc e of preferential insertion areas in this range of physical distances suggests that the Ac transposition system has differential accessabili ty to DNA with different higher order chromatin structures. This impli es that some genes may be difficult to tag, even when located very clo se to the element.