Distal regulatory functions for the uvrC gene of E. coli.

We find that the uvrC gene is preceded by three promoters (P1, P2 and P3), identified by heparin-resistant RNA polymerase-DNA complex formation, P2 and P3 promoters are located proximal to the 5' end of the uvrC gene, while the P1 promoter is separated from the uvrC structural gene by an interposed DNA region of more than 1 kb. We have reported that P2 and P3 are not sufficient to promote uvrC complementation. However, plasmids containing the direct fusion of the P1 promoter to the uvrC gene complements the uvrC defect. Insertion of IS1 downstream from the P1 promoter leads to efficient synthesis of the uvrC protein as measured in maxicells. Fusion of the lac promoter to the uvrC structural gene can substitute for in vivo regulatory functions. We conclude that uvrC protein synthesis is controlled in a complex manner and that a distal promoter, P1, is required.     

Analyses of binding sequences of the two LexA proteins of <Emphasis Type="Italic">Xanthomonas axonopodis</Emphasis> pathovar citri

Xanthomonas axonopodis pv. citri (X. axonopodis pv. citri) possesses two lexA genes, designated lexA1 and lexA2. Electrophoretic mobility shift data show that LexA1 binds to both lexA1 and lexA2 promoters, but LexA2 does not bind to the lexA1 promoter, suggesting that LexA1 and LexA2 play different roles in regulating the expression of SOS genes. In this study, we have determined that LexA2 binds to a 14-bp dyad-spacer-dyad palindromic sequence, 5'-TGTACAAATGTACA-3', located at nucleotides -41 to -28 relative to the translation start site of lexA2 of X. axonopodis pv. citri. The two spacer nucleotides in this sequence can be changed from AA to TT without affecting LexA2 binding; all other base deletions or substitutions abolish LexA2 binding. The LexA1 binding sequence in the promoter region of lexA2 is TTAGTACTAAAGTTATAA and is located at -133 to -116, and that in the lexA1 gene is AGTAGTAATACTACT located at nucleotides -19 to -5 relative to the translation start site of lexA1. Any base change in the latter sequence abolishes LexA1 binding.     

Interaction of the LexA repressor and the uvrC regulatory region

We have studied the in vitro interaction of the LexA repressor protein and the uvrC regulatory region. We find that there is specific binding to two regions, the region we have defined as lexA1 and the lexA2-lexA3 region. Our findings support the possibility of an inducible regulation for this complex operon.