Yh. Liu et al., Molecular consequences of Ds insertion into and excision from the helix-loop-helix domain of the maize R gene, GENETICS, 150(4), 1998, pp. 1639-1648
The R and B proteins of maize are required to activate the transcription of
several genes in the anthocyanin biosynthetic pathway. To determine the st
ructural requirements for R function in vivo, we are exploiting its sensiti
ve mutant phenotype to identify transposon (Ds) insertions that disrupt cri
tical domains. Here we report that the ability of the r-m1 allele to activa
te transcription of at least three structural genes is reduced to only 2% o
f wild-type activity because of a 396-bp Ds element in helix 2 of the basic
helix-loop-helix (bHLH) motif. Residual activity likely results from the s
ynthesis of a mutant protein that contains seven additional amino acids in
helix 2. This protein is encoded by a transcript where most of the Ds seque
nce has been spliced from pre-mRNA. Two phenotypic classes of stable deriva
tive alleles, very pale and extremely pale, condition <1% of wild-type acti
vity as a result of the presence of two- and three-amino-acid insertions, r
espectively, at the site of Ds excision. Localization of these mutant prote
ins to the nucleus indicates a requirement for an intact bHLH domain after
nuclear import. The fact that deletion of the entire bHLH domain has only a
minor effect on R protein activity while these small insertions virtually
abolish activity suggests that deletion of the bHLH domain may bypass a req
uirement for bHLH-mediated protein-protein interactions in the activation o
f the structural genes in the anthocyanin biosynthetic pathway.