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Chapter VII: Complementation in β-galactosidase

Agnès Ullmann, David Perrin


The term "complementation" has been generally used to define the phenomenon by which a biological function which has been lost or altered by a mutation can be restored through mutual compensation by differently altered mutants. It is generally accepted that the repair of function occurs at the level of the protein molecule and involves the interaction of differently altered polypeptide chains.

Since Benzer's definition of the cistron as the functional unit of genetic structure (Benzer, 1958), complementation tests have been used in many systems to distinguish between genes involving one or several cistrons. The distinction between inter- and intracistronic complementation is straightforward. Two cistrons should give two separate groups of complementation; the enzymes formed should be wild type, and all mutants of one group should complement all mutants of the other group. Actually the situation may be more complex (for instance, in the case of polar mutants or deletions) and in these cases, more stringent analyses are needed to draw the correct conclusion.

The purpose of studying complementation between z gene products was to determine the structure of the gene (whether it involves one or several cistrons), and thus to contribute to the elucidation of the structure of the protein controlled by the z gene. Much conflicting evidence has been reported concerning the subunit structure of β-galactosidase (i.e., whether the protomer involves one or several polypeptide chains). The study of complementation was of great help in solving this problem in a satisfactory way.

Since intracistronic complementation is mediated by...

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