With controlling elements the pattern of variegation is a reflection of the time and frequency of gene activation during kernel development resulting in differences in the size and frequency of colored sectors. This pattern of variegation is under the precise and heritable control of the regulatory element acting on the receptor elements of a two-element system. The pattern produced by a regulatory element is referred to as its state and frequent changes in state occur. With a constant receptor element, diverse En regulatory element activity can be observed.
The regulatory element can be located at many sites within the maize genome and it can transpose from one position to another. Changes in state are often associated with transposition (Peterson, Genetics, 1970; Brink and Williams, Genetics, 1973). The state of the controlling element does not appear to be an intrinsic property of the element, but due to the position of the controlling element within the maize chromosome (Peterson, in DNA Insertion Elements, 1977, CSH).
Two unstable a1 alleles (a1-m) were selected for study. Each was determined to be associated with En at this locus, thus mutability was autonomously controlled. Transpositions of En from those two unstable a1 alleles were isolated. The timing and frequency of sectoring produced by 189 transposed En's on standard responsive tester lines were visually rated against a standard set of kernels. The linkage position of each of these En's relative to a1 was determined using standard three-point linkage tests (a1 En et).
Although the two source a1-m alleles had unique and easily distinguishable patterns of variegation while the En was a component of the source a1-m allele, the transposed En's produced identical arrays of states after transposition. However, no correlation or association was found between either the timing or the frequency of sectoring and the distance from the a1 receptor allele.
There is no correlation between the distance of En from the receptor allele and the intensity of the signal (as determined by the number and frequency of colored sectors). This indicates that the diffusion of the En gene product to the receptor allele is not the limiting factor determining the state of the En. These results support the position hypothesis (Peterson, 1977) for the state of a controlling element because En's from different sources do not differ in the array of patterns that they yield following transposition.
Elaine Nowick and Peter A. Peterson
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