A Relationship between the Spm and En Control Systems in Maize

Abstract

En (Enhancer) resembles Spm (Suppressor-mutator) in controlling the mutability of the am-1 1 allele. In the absence of En or Spm, kernels possessing the am-1 1 are pale. In the presence of either of these two elements, gene action is suppressed and the am-1 1 containing kernels are colorless until a mutation event to A1 occurs (colored spots on a colorless background). En is identical to Spm in this respect; both suppress the action of the am-1 1 allele and also cause it to periodically mutate. The presence of En is determined by its simultaneous effect on am(r) 1. Evidence is presented to show that the mutation event is not a result of the loss of the regulatory element. The controlling elements of the am-1 1 and am(r) 1 alleles of the Spm and En systems, respectively, are not identical. The controlling component itself of most of the am-1 1 alleles is in a "turn off" condition in the absence of the Spm regulatory element. A similar condition has not been found in the En system. The am(r) 1 allele expresses gene action by a change in the controlling element only in the presence of the regulatory element. This discrepancy in the condition of the controlling element cannot be explained by the presence of a modified En type that has suppressor but lacks mutator qualities.