Expression of paramutable and paramutagenic alleles of the B locus --Garth I. Patterson and Vicki L. Chandler B is a regulator of the maize anthocyanin pathway. We have been studying paramutation, an unusual genetic phenomenon that occurs at the B locus. The phenomenon, as originally described by E. H. Coe, Jr., is briefly described here. The B-I allele gives intense color in many plant tissues. B', a frequently isolated, spontaneous mutant of B-I, gives faint color in the same plant tissues colored by B-I. Paramutation occurs in a B-I/B' heterozygote. When this heterozygote is crossed to a b tester, 100% of the progeny are B'/b. This invariant change of B-I to B' is called paramutation.

Figure. PolyA+ RNA was prepared from 7 B-I and 7 B' plants at various developmental stages. The RNA was slot-blotted and probed with a B-specific probe. Each sample was loaded in duplicate, and approximately 0.5 micrograms of RNA was loaded into each well. The developmental stage of the plants is indicated along the side.

Understanding the alteration in expression that leads to decreased pigmentation in B' plants might provide clues as to the nature of the genetic alteration that occurs in the B-I/B' heterozygote. As a first step, we have examined the difference in expression in B-I and B' plants. We isolated polyA+ RNA from husks of B-I and B' plants of various ages. These RNA samples were slot-blotted onto nitrocellulose and probed with a B-specific probe. We find that B-I plants have 20-100 fold more B mRNA than B' plants at each age examined, as shown in the figure. Northern blot analysis demonstrated that the B polyA+ transcripts are the same size in B-I and B' plants. These same B-I husks have accumulated 20-100 fold more anthocyanin pigment than B' husks. Pigment accumulation was measured by acid extraction and spectrophotometric determination of anthocyanin concentration. The same slot blot was stripped and reprobed with A1 and Bz1 probes (A1 and Bz1 are structural genes induced by B in husk tissue). The B-I husks also have much more mRNA from these loci. These results demonstrate that the difference in pigmentation and structural gene induction between B-I and B' correlates with a difference in mRNA accumulation. Experiments are in progress to determine if there are differences in transcription of the two alleles.


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