Family S24


Summary Holotypes Alignment Tree Genomes Structure Literature Architecture

Summary for family S24

NamePeptidase family S24 (LexA family)
Family type peptidaseS24.001 - repressor LexA (Escherichia coli), MEROPS Accession MER0000569 (peptidase unit: 85-202)
Content of familyPeptidase family S24 contains two-domain proteins that undergo autolysis, separating the functional domains.
History Identifier created: Methods Enzymol. 244:19-61 (1994)
Catalytic typeSerine
Active site residuesS119 K156 
Active siteThe active site consists of a Ser, Lys catalytic dyad much more widely spaced than in the serine-type D-Ala-D-Ala carboxypeptidases (see clan SE). The nucleophilic hydroxyl group of the serine attacks the si-face of the peptide bond rather than the re-face, which is the usual mode of attack in serine peptidases (Paetzel & Strynadka, 1999).
Activities and specificitiesThe endopeptidases in family S24 perform only one cleavage: autolysis which separates the DNA-binding domain from the rest of the protein. Cleavage occurs at an AlaGly or a CysGly bond. In repressor LexA (S24.001), cleavage only occurs when LexA associates with the RecA protein. LexA occurs in two conformations, cleavable and non-cleavable. In the cleavable form, a large conformation change occurs placing the cleavage site adjacent to the active site. The non-cleavable state is more stable, but the cleavable form is stabilized by the RecA protein (Luo et al., 2001).
Molecular structureThe tertiary structures of several members of the family have now been determined, including the cleaved form of the UmuD protein (S24.003; Peat et al., 1996), and native and cleaved forms of repressor LexA (Luo et al., 2001). The proteins consist mainly of beta structure, in contrast to the serine-type D-Ala-D-Ala carboxypeptidases which are alpha/beta proteins. UmuD exists in solution as a homodimer.
Basis of clan assignmentType family of clan SF.
Distribution of family Bacteria details  
Archaea details  
Protozoa details  
Fungi -  
Plants details  
Animals details  
Viruses details  
Biological functionsRepressor LexA is involved in the SOS response leading to the repair of single-stranded DNA within the bacterial cell. Single-stranded DNA activates the RecA protein, which in turn mediated LexA cleavage. Cleaved LexA no longer acts as a repressor, which means that several LexA-controlled genes are switched on. These genes are involved in DNA repair and other cell survival processes. UmuD is also involved in the SOS response, but here cleavage releases and activates the DNA polymerase UmuC which forms a complex with uncleaved UmuD (Reuven et al., 1999). Cleavage of the bacteriophage lambda repressor CI leads to prophage induction: the prophage is integrated into the host genome, but expession is blocked by repressor CI until the SOS response leads to CI autolysis, derepression of the prophage and expression of the lytic genes and growth of the virus (Little, 2004).
Statistics for family S24Sequences:14488
Identifiers with PDB entries:4
Downloadable files Sequence library (FastA format)
Sequence alignment (FastA format)
Phylogenetic tree (Newick format)
Other databases CATH
PFAM PF00717
SCOP 51307
Peptidases and Homologues MEROPS ID Structure
repressor LexAS24.001Yes
phage lambda repressor proteinS24.002Yes
UmuD proteinS24.003Yes
RvuZ homologue protein (Rattus)S24.004-
irvR self-cleaving proteinS24.005-
repressor protein phage e14 (Escherichia coli)S24.A20-
Family S24 non-peptidase homologuesnon-peptidase homologue-
Family S24 unassigned peptidasesunassignedYes