Difference between revisions of "Part:BBa K3788017"

Latest revision as of 14:56, 21 October 2021

lexA-repressed promoter

This part contains the LexA-responsive promoter region of the Colicin A gene, amplified from the Colicin A encoding plasmid (pRL1 or pColA).

Background information

LexA is a transcriptional regulator that binds to the promoter regions of the numerous SOS-response regulatory genes, repressing the expression of the downstream genes. The consensus LexA binding box is CTG-N10-CAG. In response to the presence of DNA lesions (resulting from UV or mitomycin treament for exemple), the RecA protein is activated and stimulates the autocatalytic digestion of LexA, leading to the derepression of the promoter.

Colicin A is a plasmid-encoded toxin produced by some E. coli strains and active against other non-immune E.colistrains. Colicin A is encoded by the caa gene. The toxin production is induced by the SOS response. caa was demonstrated to be transcribed as an operon with the cal gene, encoding the lysis protein CaL (see BBa_K3788018, BBa_K3788016 and BBa_K3788015 for details on CaL). Upon induction of the SOS system, Colicin A accumulates in the cell cytoplasm. The delayed production of the lysis protein results in a time-controlled lysis of the cells, and the release of colicin A in the environment. An immunity protein encoded by the caigene is transcribed constitutively in the opposite direction of the caa-cal operon, insuring immunity of the pColA plasmid carrying strain from the lethal activity of the toxin.

The caa promoter region:

The -35 and -10 promoter elements, the +1 transcription site and LexA binding sites have been identified in the promoter region of caa.

Binding of LexA on the caa promoter was experimentaly demonstrated by footprinting experiments (1). LexA protein binds with a high affinity to an approximately 40 bp long sequence just downstream of the -10 element. The sequence of the binding site includes two overlapped SOS boxes.

Figure adapted from: Lloubes et al., Nucleic Acids Research 1986 (DOI: 10.1093/nar/14.6.2621). Organization of the caa-cal operon and the cai gene. The nucleotide sequences for the promoters are presented: the recognition and binding sequences for RNA polymerase are presented as yellow boxed(-35 and -10 elements). SA indicates the transcriptional start site for the caa-cal operon. Arrows above the start site indicates the direction of transcription, the bold line indicates the major band detected by SI mapping. The binding site for LexA in the operator region for caa is indicated by brackets overlining the protected region for the sense-strand and brackets underlining the sequence (not represented) for the anti-sense strand. The two overlapped consensus "SOS sequences" are presented as a pink box.

Sequence and Features