COLICIN-U, A NOVEL COLICIN PRODUCED BY SHIGELLA-BOYDII

A novel colicin, designated colicin U, was found in two Shigella boydi i strains of serovars 1 and 8. Colicin U aas active against bacterial strains of the genera Escherichia and Shigella. Plasmid pColU (7.3 kb) of the colicinogenic strain S. boydii M592 (serovar 8) was sequenced, and three colicin genes were identified. The colicin U activity gene, cua, encodes a protein of 619 amino acids (M-r, 66,289); the immunity gene, cui, encodes a protein of 174 amino acids (M-r, 20,688); and th e lytic protein gene, cul, encodes a polypeptide of 45 amino acids (M- r, 4,672). Colicin U displays sequence similarities to various colicin s. The N-terminal sequence of 130 amino acids has 54% identity to the N-terminal sequence of bacteriocin 28b produced by Serratia marcescens . Furthermore, the N-terminal 36 amino acids have striking sequence id entity (83%) to colicin A. Although the C-terminal pore-forming sequen ce of colicin U shows the highest degree of identity (73%) to the pore -forming C-terminal sequence of colicin B, the immunity protein, which interacts with the same region, displays a higher degree of sequence similarity to the immunity protein of colicin A (45%) than to the immu nity protein of colicin B (30.5%), Immunity specificity is probably co nferred by a short sequence from residues 571 to residue 599 of colici n U; this sequence is not similar to that of colicin B. We showed that binding of colicin U to sensitive cells is mediated by the OmpA prote in, the OmpF porin, and core lipopolysaccharide. Uptake of colicin U w as dependent on the TolA, -B, -Q, and -R proteins. pColU is homologous to plasmid pSB41 (4.1 kb) except for the colicin genes on pColU. pSB4 1 and pColU coexist in S. boydii strains and can be cotransformed into Escherichia coil, and both plasmids are homologous to pColE1.