Regulation of Gene Action — Gene Control Systems in Maize 



469 



controlled. It has been found, however, 

 that the relative frequency with which Mp 

 transposes away from P' Mp is 1 00 in the 

 absence of a transposed Mp; about 60 in 

 the presence of one transposed Mp; and 

 about 5 in the presence of two transposed 

 Mp's. Thus, the transposition of Mp from 

 P' Mp is controlled by the presence of trans- 

 posed Mp. 



Topographical Relations of Controlling 

 Elements - 



In the Ac-Ds system, Ac controls Ds not 

 only by regulating Ds transposition by break- 

 age or some other mechanism which may 

 involve contact, but in other ways, as de- 

 tected by the kind of phenotypic effect Ds 

 produces on its linear gene neighbor to one 

 side. The capacity for transposition is pos- 

 sessed by both the regulator gene {Ac) and 

 the operator gene (Ds), a feature unknown 

 in the bacterial systems. 



In bacterial systems the regulator and 

 operator genes may be close to each other or 

 they may be a considerable map distance 

 apart. In view of the transposability of both 

 elements in maize, it can be hypothesized 

 that both elements are at times adjacent or in 

 close linkage. Consequently, P' Mp can be 

 interpreted P' op }I " R Mp where op iIp is the 

 operator and R Mp the regulator gene. In 

 general, it can be hypothesized that both an 

 operator gene and a regulator gene are lo- 

 cated close to a structural gene when only 

 the presence of the regulator gene is known 

 with certainty. 



This general hypothesis is tested in the 

 following way. The gene for bronze, Bz, 

 is located in chromosome 9 and has a com- 

 pletely recessive allele, bz, which produces 

 no color. If a transposition of Ac occurs 

 near Bz, resulting in a variegated bronze 

 color, this locus is now assumed to be 

 Bz op Ac R Ac . If a two-element control sys- 



2 This section is based upon the work of B. Mc- 

 Clintock (1961, 1962, 1963). 



PERCENT RECOMBINATION 

 P - Mp 



2.6 



4.3 



7.6 

 12.0 

 42.0 



VARIEGATED SECTORS 

 PER 1000 KERNELS 



15 

 11 



8 



0.2 



figure 37-7. Effect of distance of Mp from 

 P upon transposition rate of Mp to P. 



tern actually exists at this locus, three kinds 

 of subsequent transpositional events are 

 possible: 



1. Transposition of both op Ac and R u . 

 (This operation should release Bz from 

 the control system, and R Ac should 

 prove absent from the vicinity of Bz.) 



2. Transposition of op Ar only. (This 

 should also release the Bz gene from 

 control by R Ar ; but the latter should 

 still be near Bz and capable of regulat- 

 ing op Ac located at other sites in the 

 genotype.) 



3. Transposition of R Ac only. (This 

 should leave the Bz op Ar locus still 

 under the control of R Ac in its new 

 location. ) 



Experimental results confirm these expec- 

 tations. (Were there a one-element control 

 system, the third alternative could not oc- 

 cur.) Since other results demonstrate that 

 R ac R m p and st}n other R genes are dif _ 



ferent (each regulates its own type of op 

 gene), numerous and different op-R systems 

 occur in maize. The transposability of op 

 explains why a given op can become the 

 operator gene of a variety of loci previously 

 uncontrolled by that op. Additional work 

 with corn is needed to determine: 



1. Whether transposition of op or R in- 

 volves an addition or a replacement 

 of an already present op or R locus 



