Ds at the shrunken locus

We have continued our study of clones obtained from genomic DNA of mutants sh-m5933 and sh-m6233. The following results have been obtained:

1. Both clones contain one segment homologous by hybridization and restriction analysis to a corresponding region in the clone derived from Sh DNA. This segment extends to a certain point (breakpoint). Beyond this point, the DNA extending to the end of the insert in the lambda phage does not show hybridization to the wild type-derived clone.

2. The breakpoints are located 2.5 kb apart. The breakpoint in sh-m5933 seems to be located in an intron. The breakpoint in sh-m6233 is located upstream and may be located outside the transcription unit.

3. The "foreign" DNAs adjacent to the breakpoints hybridize to each other, but differ in restriction patterns.

4. On sh-m5933 "foreign" DNA, two pairs of inverted repeats are present. These interdigitate with each other. One member of one pair terminates exactly at the breakpoint.

5. DNA sequence analysis of the repeat bordering at the junction in sh-m5933 detects a region which contains repeats of the hexanucleotide CCGTTT and derivatives thereof. These are oriented either directly or inverted to each other.

6. The DNA bordering directly at the junction has been subcloned and used as a probe for hybridization to genomic DNA. Up to 40 bands are revealed. The structure of these bands is heterogeneous.

7. If the DNA near the junction is Ds DNA, several non-identical copies of this element are present within genomic DNA of several maize lines. These show difference in restriction patterns.

Addendum: In the last issue of MNL, we reported the isolation of an unstable Adh1 mutant, which we assumed to be caused by a controlling element different from the Ds-Ac system. Further genetic analysis of the mutant, however, showed that the instability of the Adh1 gene expression is linked to the presence of an Ac element.

U. Courage-Tebbe, H.-P. Doring, P. Starlinger, E. Tillmann and E. Weck


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