Fig. 1
From: Development and progress for three decades in umu test systems
Schema showing the principle of umu test using umuC”lacZ fusion gene. In uninduced cells, the LexA repressor protein, acts to repress by binding to operator sequences (called an SOS box) upstream from umu operon. On the other hand, in induced cells: when the DNA is damaged by genotoxins, or when replication is blocked by various ways, the cell induces an SOS signal. The SOS signal activates a coprotease activity of the RecA protein, and this protease (RecA filaments) activate the autocleavage of the LexA repressor, allowing umuC” lacZ fusion gene expression, and the chimeric UmuC”LacZ fused protein is produced. Since the umuC gene is fused with the lacZ gene for β-galactosidase activity, the induction of umuC gene can be estimated by determination of the β-galactosidase activity