An Escherichia coli K-12 strain carrying rbsB103 signal sequence point mutation accumulates export-defective precursor of ribose-binding protein in the cytoplasm, which results in the defect of transport as well as tactic response to ribose.
Spontaneous pseudorevertants altered in the export of ribose binding protein were obtained in rbsB103 mutant by searching for mutants that showed an altered growth and tactic response to ribose. Genetic analysis of pseudorevertants revealed that the reversions were resulted from intragenic or extragenic suppression.
Six mutants harboring intragenic suppressor mutation exhibit full restoration of export to the mutant RBP on the other hand, two mutants harboring extragenic suppressor mutation show partial restoration of export. One intragenic suppressor showed the decreased rate of migration of the RBP during SDS-PAGE that may be caused by a second mutation in mature part of RBP.
Mapping experiments showed that at least an extragenic suppressor phenotype is not the result of mutation in prlA(sec Y) gene. But, this result does not exclude the possibility that PrlA protein is involved in RBP export.
We have analysed the export kinetics of the RBP in strains harboring extragenic suppressor mutation and harboring a second mutation in mature part of RBP. Our results suggest that both these unknown gene products and the export competent state of RBP may play an important role in the export of RBP to the cytoplasm.