University of Missouri, Agricultural Experiment Station
Abstract
Extensive reorganization of components of the genome is initiated in maize by the breakage-fusion-bridge (BFB) cycle. Displacements include chromosomes other than those undergoing the cycle. Restructuring ranges from those rearrangements that are readily viewed with the light microscope, such as reciprocal translocations, inversions, duplications, etc., to apparently short DNA segments that include components of transposable gene control systems. Much of the restructuring appears to be nonrandom. Some examples of this are conspicuous, such as centromere to centromere, knob to knob, or centromere to knob attachments. Others are exhibited by attachment of a segment of some chromo some to a newly broken end o f a chromosome, or the placement of a piece of one chromosome onto the end of another chromosome that had not undergone the BFB cycle. These modifications suggest participation of "restriction enzyme" systems in their formation. One system responsible for special types of chromosomal modifications was extensively examined. The chromosome location of its principal genetic element was determined. This genetic element has never given evidence of serving as a component in a gene-control system. Instead, it serves to cut chromosomes. The locations of these cuts do not appear to be random. The BFB cycles exposed the presence in the maize genome of transposable elements that can serve to control the type and time of gene action. Previous to undergoing the cycle, these elements were quiescent in the genome. One component of a gene control system produces a product that is responsible for trans position of the elements of a system. In this regard, each system operates quite independently of the other. The product can induce modifications in chromosome organizations that are by-products of the transposition mechanism. It also can cause receptive elements located in different regions of the genome to respond by inducing DNA modifications in situ and without altering their subsequent receptivity to the product. Or, the product may cause an element to modify the organization of chromatin to one or the other side of it. Some responses of a receptive element that is located within a gene locus result in removal of sensitivity to the inducing product. Stable, new alleles are so produced. In such instances, the gene product may be altered and the patter n of its expression may also be altered during development of plant and kernel. It is suspected that the BFB cycle initiates stress within the genome and that the stress calls up reserves to counteract it. In some instances, coping with stress may involve a simple solution, such as gene amplification. Coping with drastic types of stress may initiate seemingly disorderly types of response. It is conceivable that, in some instances, stabilization may follow such disorder. This could provide newly organized genomes with orderly operating gene-control systems while still retaining those components that again can respond to stress.BARBARA McCLINTOCK Carnegie Institution of Washington, Cold Spring Harbor Laboratory, Cold Spring Harbor, N .Y
