344 CELL HEREDITY 



This view has been supported and confirmed in studies by Brink and 

 colleagues, who reinvestigated the variegated pericarp system. R. A. 

 Emerson, who initiated the study of mutable genes in maize, had investi- 

 gated a P^^ (variegated pericarp) allele and concluded that the mutability 

 was an intrinsic property of it. Brink et al. later showed that the muta- 

 ble allele is a complex consisting of the top dominant allele P" and Mp. Mp 

 is a controlling element very similar to Ac if not identical with it. The 

 presence of Mp at the F locus suppresses the expression of F"^, and muta- 

 tion from variegated to stable red involves the transposition of Mp out of 

 the P locus, frequently to a neighboring site, but occasionally to other 

 chromosomes. The finding of just one Mp element in this material, 

 before as well as after mutation and regardless of position, is strong evi- 

 dence that Mp (and Ac as well) are physical entities which can move 

 from site to site, rather than representing merely altered responses or 

 activities. 



The meaning of the transposition phenomenon is totally unclear at this 

 time. It is not known whether mutability always involves a transposi- 

 tion, since there is no foolproof method for detecting transpositions in 

 every system. However, this aspect of the findings does bear a formal 

 resemblance to the following recent observations on episomes in the 

 E. coli system. 



As may be recalled from Chapter 5, in some strains of E. coli, the 

 phage X is present, in its prophage form, located close to the gal region. 

 This region controls the formation of three enzymes involved in the 

 metabolism of galactose, which are inducible and repressible. Whether 

 X is carried as prophage, multiplying with the chromosome, or whether 

 it becomes vegetative, multiplying freely and lysing the cell, depends 

 upon what has been called the repression of X. The prophage state 

 is considered a condition in which free phage multiplication is repressed. 

 This repression can be lifted by UV-irradiation of the cells. In cells 

 which are also repressed with respect to synthesis of the gal enzymes, 

 UV-induction of X also releases their repression, and there is a burst 

 of enzyme formation in the cells before lysis. The specificity of the 

 response is shown by the fact that induction of a different prophage, 

 located further from the gal region, does not affect the gal enzymes. 



Whether there are common features in mechanism which go beyond a 

 formal similarity between the E. coli and maize systems in respect to 

 release of repression by transposition is a problem for the future. There 

 are other aspects of the maize system which may be of more direct rele- 

 vance to the central theme of this chapter. These aspects concern the 

 position effect itself, that is, the ability of controlling elements by their 

 location to determine the expressibility of neighboring genes. 



