rbsB103, a mutational alteration in the signal sequence of ribose binding protein(RBP) of Escherichia coli blocks completely the export of the protein to the periplasm. Intragenic suppressors for this mutation have been selected on minimal medium with ribose as a sole carbon source. The suppressor mutations were found to have single amino acid substitutions in the mature portion of RBP, which resulted in the mobility shift of the protein on SDS polyacryamide gel electrophoresis. Two suppressor changes so far examined were reported to have decreased rates of folding of the purified proteins as monitored by flourescence spectroscopy (Teschke et al, 1990). Amino acid changes of the suppressors were localized in several peptides which are packed to form the N terminal domain of typical bilobate conformation of RBP. The location of all suppresor changes in N domain is striking since they were isolated at random. It is likely that the amino acid residues where the suppressor changes have been found are critical in folding of the protein to be translocated into periplasm. A new mutational change in the signal sequence of RBP, rbsB(-14) AE, was introduced by site directed mutgenesis, which reduces export efficiency at about seventy percent. The suppressor mutations for rbsB103 overcomed the export defect of rbsB(-14)AE. It seems that the mode of suppression is not a direct interaction with signal sequence mutation since no strict allele-specificity was observed.
The involvement of SecB, molecular chaperone, which is known not to be involved in the translocation of RBP, was investigated in the suppression of signal sequence mutation. Translocation efficency was found to be increased by the presence of SeoB for all suppressors with rbsB103 or rbsB(-14)AE. Folding characteristics of RBP altered by the suppressor changes might affect the affinity of interaction between SecB and RBP.
