LexA and lambda Cl repressors as enzymes: specific cleavage in an intermolecular reaction

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• The SOS response in bacteria involves the inactivation of LexA repressor through specific cleavage.
• RecA protein is essential in vivo for LexA cleavage, acting indirectly as a coprotease by enhancing LexA's self-cleavage activity.
• Cleavage of lambda Cl repressor, a similar process, leads to prophage induction but occurs at a much slower rate.

Purpose of the Study:

• To investigate the mechanism of LexA repressor self-cleavage.
• To characterize the intermolecular cleavage activity of the LexA C-terminal fragment.
• To compare the enzymatic activity of LexA and lambda Cl repressors on LexA substrates.

Main Methods:

• Describing an intermolecular cleavage reaction.
• Utilizing LexA and lambda Cl repressor fragments.
• Assessing cleavage efficiency on LexA substrates.

Main Results:

• The C-terminal fragment of LexA was found to act as an enzyme, cleaving other LexA molecules.
• The C-terminal fragment of lambda repressor demonstrated efficient cleavage of LexA substrates, comparable to the LexA enzyme.
• This suggests that the slow self-cleavage rate of lambda Cl repressor is due to a weak interaction between its cleavage site and the active site.

Conclusions:

• LexA repressor possesses an inherent intermolecular self-cleavage activity mediated by its C-terminal fragment.
• The lambda Cl repressor shares a similar cleavage mechanism, highlighting conserved enzymatic properties.
• Differences in cleavage rates are attributed to variations in active site-cleavage site interactions, providing mechanistic insights into DNA repair and phage regulation.