Ds controlling elements of maize at the shrunken locus are large and dissimilar insertions

A recombinant clone for the wild-type gene of sucrose synthetase has been selected. Subclones of the genomic clone were made and used as probes to construct a restriction map of the wild-type locus. The transcribed region and the direction of transcription in the map were also determined. Four Ds-induced mutations of the locus that were isolated by B. McClintock were analyzed and partially mapped. Two mutations contain insertions of ∼20 kb of foreign DNA into the transcribed region; two others are at the 5′ end of the gene. There are profound differences in the restriction maps of the Ds elements in the mutants analyzed, even though these elements have a common genetic origin and are separated by only a few generations. Comparison of one pair of closely related Ds elements indicates that these differences may be the result of extensive internal rearrangements. Exchange with other elements in the genome remains an alternative possibility. A revertant of one of the Ds-induced mutations has maintained a 21–22 kb insert at the same location in the transcribed region. This insert differs from its predecessor by extensive rearrangement in two thirds of its length. Part, or all, of this rearrangement presumably permits functional expression of the gene.