sh-m6258 is one of the mutable sh alleles which arose when B. McClintock analysed transposition events of the Ds element located between C-I and Sh. Biochemical analysis of the sh-m6258 allele shows that the Sh gene is interrupted by a long insertion of at least 45 kb. Cloning of part of this insertion including the 5' and 3' junction between Sh and insert DNA detects Ds elements on both sides of the insert. Structurally this insertion in sh-m6258 is similar, though not identical, to the 30 kb insert found in the sh-m5933 allele. Interestingly, the sh-m5933 allele has the same genetical progenitor as the sh-m6258 allele.

On one side of the insert of sh-m6258 we find a single Ds element of approximately 2 kb. The other side carries a double Ds structure at the junction with the Sh DNA. This structure comprises one complete 2 kb Ds element and one half Ds element. The same construct was found on one side of the above-mentioned sh-m5933 insert. This indicates to us that both alleles were formed by the same mechanism.

Cloning and partial DNA sequencing of a revertant allele of sh-m6258 (isolated by N. Fedoroff, Baltimore) reveals the presence of a 2 kb Ds element at the location of the long insert described above. We are presently sequencing the Ds parts of the sh-m6258 allele in order to understand what kind of excision or other rearrangement gave rise to the remaining Ds element in the revertant allele. The insertion site is located in the last but one intervening sequence of the sucrose synthase(Sh) gene. Thus, the 2 kb insert in the revertant does not necessarily interfere with gene expression.

Ingrid Pahl, Maria Durany and Hans-Peter Doring

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