Molecular studies on mutations at the Shrunken locus in maize caused by the controlling element Ds

The structure and expression of the Shrunken (Sh) locus have been examined in several maize strains with mutations at the locus caused by the controlling element Dissociation (Ds). Three of the strains (sh-m6233, sh-m5933, and sh-m6258) are of independent origin, and the fourth (sh-m6795) represents a spontaneous recessive sh allele derived from an Sh revertant of strain sh-m6258. The sh-m6233 and sh-m5933 strains produce undetectable levels of the Sh-encoded sucrose synthetase and very small amounts (less than 1%) of an apparently normal sucrose synthetase mRNA in immature endosperm tissue. Both strains have rearrangements affecting the structure of the locus near the 5' end of the transcription unit. The Sh locus in the related strains sh-m6258 and sh-m6795 is interrupted by an insertion or a rearrangement having one breakpoint in an intervening sequence near the 3' end of the mRNA coding sequence. The 5' end of the gene is transcribed in immature kernels of both mutants, giving aberrant poly (A)+ mRNAs that are homologous to the normal transcript up to the insertion or rearrangement breakpoint and lack homology to the 3' end of the gene. The aberrant transcripts from both strains are translated in vivo and in vitro, yielding 82- and 85-kD proteins that are immunologically related to the 92-kD Sh-encoded sucrose synthetase monomer.