For the purpose of the present discussion, "recombination events" are defined broadly to include all events that are promoted by the confronta
tion of two different chromosomes or D N A molecules or different parts of a single chromosome. For some "recombination events," homology and presumably homologous pairing between the participants may be an essential prerequisite. For others, little or no homology may be needed.
Potentially, both kinds of events may generate deletions. In cases where homology is essential, the crucial deletion-promoting event would be a mispairing reaction which would then be followed by an unequal cross
over (Fig. 7c). Lerman (143) has hypothesized that the intercalation of acridines between adjacent base pairs promotes unequal crossovers through mistakes in homologous pairing. In some organisms, a correlation has been observed between acridine-induced frame-shift mutations and general recombination potential (144, 14&)- In other organisms, such a correlation has not been found (48, 146). If a certain kind of deletion is promoted by unequal crossovers, one expects these deletions to occur preferentially in those regions of a chromosome where there is sequence similarity. It is clear that some regions of a bacterial chromosome are more prone to deletions than others (e.g., region in the vicinity of cys in Salmonella or trp in E. coli). Within experimental limits, there has been nothing to suggest that these regions have repeating or similar nucleotide sequences. On the contrary, there is now sufficient evidence (reviewed in 72a, 75b, 147) that two chromosomes that share little or no detectable homology may participate in recombination-type breaks and restitutions. Examples are the prophage integration and deintegra-tion events promoted by the int funcdeintegra-tion of phage λ and the transideintegra-tion of the F factor from the integrated to the autonomous state. It is also known that such events can sometimes involve the deletion from the chromosome of genes adjacent to the chromosomal attachment site of the episome. The adjacent genes may be on one or both sides of the
inserted episome and, in the latter case, the breaks and restitutions need involve only chromosomal material. As a working hypothesis, it is attractive to postulate that events of this type may be promoted by episomic elements that are otherwise genetically "silent," that is, elements that have the capacity for chromosomal integration-deintegra-tion but are not otherwise easily recognizable or, at any rate, have not as yet been recognized. This would explain why the frequency of deletion mutants is independent of generalized recombination functions such as those determined by the rec and red genes (H8, 149). In the vicinity of a region that is prone to deletions such as cys in the Sal
monella chromosome, there then ought to be one or more sequences that either directly or indirectly serve to recognize these hypothetical episomes.
VIII. CONCLUSIONS
Genetic deletions are very rare events (about 10~7 or less per cell generation) but a variety of selective techniques can be and has been devised for the isolation of deletion mutants. The unambiguous way in which deletion mutants can be exploited in topological fine-structure mapping fully justifies attention to their isolation and use. Available evidence suggests that such mutants will occur in virtually all regions of a chromosome. The frequency with which they occur will vary with the genetic region as well as the strain involved.
The mechanisms by which deletions are produced are not understood.
The rarity of the events that promote them have made indirect ap
proaches to the question of the mechanisms of their origin necessary.
Some of these indirect approaches suggest that the generation of deletions can be associated with events promoting the deintegration of episomic elements from a chromosome.
It seems reasonable to believe that enzymes identical or similar to the ones that are already described and are known to cut or repair D N A have a role in generating deletions. Mutations in genes controlling some of these enzymes (e.g., D N A polymerases and ligases) may affect deletions, but there is as yet no evidence to suggest that the functions mediated by these known enzymes or any other function are essential for generating deletions.
Note added in proof: Since this chapter was written, pertinent informa
tion has become available and references to this literature are indicated
here, (a) There is now every indication that the technique of physically mapping D N A molecules (Section IIIC) can be extended to genetic elements besides viral D N A {150-152). (b) New enzymes or enzymatic activities involving D N A as the substrate have been discovered in E. coli.
In some cases, the genes determining them have been identified (153-160). (c) Novel and rare types of interactions between plasmids or a plasmid and chromosome are being observed (161-163). There is a recent and interesting volume on bacteriophage lambda (164); in particular, the article by N. Franklin relates to some of the questions raised here.
ACKNOWLEDGMENTS
Research has been supported by Grant A4429 from the National Research Council of Canada. During the preparation of this chapter, I have received thoughtful criticism from colleagues in this department and from J. Drake, P. Howard-Flanders, R. Iyer, and W. Szybalski.
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