Mutant sh-m6233 belongs to the series of mutable shrunken alleles caused by the insertion of Ds that were isolated and characterized by McClintock. We have studied the DNA of the mutant by the blotting technique and found that a 4 kb DNA insertion is present in the sucrose synthase gene. The preliminary restriction map of the inserted DNA is indistinguishable from the restriction map of the "double Ds" described in the two preceding reports. However, the orientation of the sh-m6233 Ds structure is opposite to that of the 4 kb Ds sequenced from sh-m5933.
We have not yet cloned the whole foreign insert of sh-m6233, but have isolated a clone carrying part of the insertion and part of the shrunken gene. This allowed us to determine the exact point of insertion. It is located 2.5 kb towards the transcription start relative to the insertion site in sh-m5933. The sequence adjacent to the insertion site begins with the same 11 bp that form inverted repeats at the termini of the double Ds structure in sh-m5933.
These results do not support the assumption that the Ds elements causing mutations sh-m5933 and sh-m6233 are non-transposable, as was originally suggested by B. McClintock. The assumption that mutable sh alleles are caused by a *nontransposable" Ds was partly derived from the observation that phenotypic revertants to Sh wild type are unstable themselves and give rise to new mutable sh alleles at considerable frequency. It will be interesting to see whether the phenotypic revertants of sh-m6233 arise by transposition of Ds to a nearby site. Short range transposition has been described by Brink for transposable element Mp (which is functionally identical to Ac) at the P locus.
E. Weck,* U. Courage-Tebbe, H. P. Doring and P. Starlinger
*Advanced Genetic Sciences, Inc., Oakland, CA
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