WORKED PROBLEMS

Problem

The b1 allele encodes a transcription factor that stimulates production of anthocyanin, a purple pigment in plants. What would be the effect of deleting the seven tandem repeats that are located 100,000 bp upstream of the b1 locus in maize?

Solution Strategy

What information is required in your answer to the problem?

The effects of deleting the repeats.

What information is provided to solve the problem?

• The b1 locus encodes a transcription factor that stimulates anthocyanin production in plants.
• The seven tandem repeats are located upstream of the b1 locus.

For help with this problem, review:

Paramutation in Corn in Section 21.3.

Solution Steps

The information provided in the text indicates that the B-I B-I plant normally has high expression of the b1 locus, produces anthocyanin, and is purple in color. The tandem repeats are required for high transcription of the b1 locus, which encodes a transcription factor that stimulates production of anthocyanin, a purple pigment. The tandem repeats act like an enhancer, stimulating transcription of the b1 locus in the B-I B-I genotype. Without the enhancer-like action of the repeats, b1 will be transcribed at minimal levels and little anthocyanin will be produced. This will result in lightly pigmented B-I B-I plants, the same phenotype as is usually seen in plants with genotype B' B' plants.

The tandem repeats also encode 25 nucleotide-long siRNAs, which are required for paramutation (conversation of the B-I allele into B′* alleles). Deletion of the tandem repeats would thus eliminate the ability of B alleles to carry out paramutation.