LexA repressor

The LexA repressor (Mr 22,700) inhibits transcription of all the SOS genes (Fig. 28-22), and induction of the SOS response requires removal of LexA. This is not a simple dissociation from DNA in response to binding of a small molecule, as in the regulation of the lac operon described above. Instead, the LexA repressor is... 

Among these are the well-known E. coli leader peptidase355 356 and other signal peptidases.357 These are integral membrane proteins that cleave N-terminal signal sequences from proteins incorporated into plasma membranes. Another enzyme of this class is the lexA repressor and protease discussed in Chapter 28. 

Figure 22.22. SOS response in E. coli. Under normal growth conditions (SOS off), genes under the SOS control are repressed by the LexA repressor. DNA damage or replication block triggers SOS response, leading to activation of the RecA co-protease and subsequent inactivation of the LexA repressor by RecA-assisted auto cleavage. This results in induced transcription of the various SOS genes (SOS on). Combined cellular activities such as DNA repair and translesion synthesis eventually removes the SOS signal. Consequently, the RecA co-protease is inactivated and the LexA repressor is accumulated in cells, returning cells to the SOS off state.

During normal growth, the SOS genes are negatively regulated by LexA repressor protein dimers LexA binds to a 20-bp consensus sequence (the SOS box) in the operator region for those genes. 

An example for autoregulation is found in the hut-operon of E. coli and in the regulation of the SOS response in bacteria via the lexA repressor. There are examples of autoregulation at the level of translation as well (see Section 1.5.5.1). 

Fifth, because the currently known modifications of PCNA are on the same Lys residue and do not signal for degradation, it can be inferred that a reversible process may play an important role in the restoration of eukaryotic PRR. Although it is known that desumoylation of PCNA probably requires Ulpl (Hoege et al, 2002), a Ub protease involved in deubi-quitination of PCNA has not been reported. Similar to the negative feedback by the LexA repressor in the E. coli SOS response (Fig. 1), the recovery from DNA damage in eukaryotes might result in the concomitant sumoylation of PCNA to inhibit PRR. 

Yu, X., and Egelman, E. H. (1993). The LexA repressor binds within the deep helical groove of the activated RecA filament. J. Mol Biol 231, 29-40. 

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