An Escherichia coli membrane protein with a unique signal sequence

The smp gene, whose promoter divergently overlaps with the Escherichia coli serB promoter, encodes a 24-kDa membrane protein of unknown function. An smp fusion to the lacZ gene was constructed to select for smp promoter up-mutations. The mutations isolated, however, were located within the smp structural gene rather than within the promoter region. A 15-bp deletion and two different point mutations, corresponding to the hydrophobic region of the Smp signal sequence, resulted in a Lac+ phenotype. The point mutations introduced a positively charged amino acid into the hydrophobic region. The Smp signal sequence is unique in that it is encoded by an mRNA which has the potential to form a 9-bp hairpin structure with a 39-nt loop, that conforms to the idealized mirror symmetric sequence GC(RGUGR)(YUGUY)5 (RGUGR)CG. This highly symmetrical region may encode additional intragenic information important for the expression of smp. A computer search revealed that the smp gene product shares homology with elongation factor Ts and ribosomal subunit L4, two components of the E. coli translational apparatus.