Abstract
Systematic sequencing of the Escherichia coli K-12 chromosome (GenBank entry U18997) has revealed the presence of an apparently complete operon of genes (the gspC-0 operon) similar to genes coding for components of the main terminal branch of the general secretory pathway (e.g., the Klebsiella oxytoca pulC-0 pullulanase secretion operon) and to related genes required for type IV pilus biogenesis. For example, the last gene in the gsp operon, gspO (formerly hopD), encodes a protein which is similar to several type IV prepilin peptidases. Expression of gspO from lacZp promotes cleavage of two known prepilin peptidase substrates in E. coli K-12: Neisseria gonorrhoeae type IV prepilin and K. oxytoca prePulG protein. gspO also complements a mutation in the corresponding gene (pulO) of the pullulanase secretion operon when it is expressed from lacZp. Another gene in the gsp operon, gspG (formerly hopG), encodes a protein similar to prePulG, a component of the pullulanase secretion pathway. Expression of gspG from lacZp leads to production of a protein which (i) is recognized by PulG-specific antiserum (and by antiserum against the Pseudomonas aeruginosa PulG homolog XcpG [formerly XcpT]), (ii) is processed in cells expressing gspO, and (iii) restores secretion in cells carrying a pulG mutation. The chromosomal copies of gspG and gspO are apparently not expressed, probably because of very weak transcription from the upstream region, as measured by using a chromosomal gspC-lacZ operon fusion. Thus, the gsp operon of E. coli K-12 includes at least two functional genes which, together with the rest of the operon, are probably not expressed under laboratory conditions.
Highlights
Several species of gram-negative bacteria secrete extracellular proteins via a two-step general secretory pathway (GSP) which begins with Sec-dependent, signal peptide-mediated translocation across the cytoplasmic membrane
In view of the closer similarity of these E. coli genes to the genes coding for GSPMTB components than to genes required for type IV piliation, we propose that they be renamed gsp
Coli K-12 gsp genes could be functional is that expression of the N. gonorrhoeae gene for the precursor of type IV pilin in E. coli K-12 results in the production of pilin which comigrates with mature pilin, possibly because of processing by gspOencoded prepilin peptidase [7, 12]
Summary
Several species of gram-negative bacteria secrete extracellular proteins via a two-step general secretory pathway (GSP) which begins with Sec-dependent, signal peptide-mediated translocation across the cytoplasmic membrane. Coli K-12 gsp genes could be functional is that expression of the N. gonorrhoeae gene for the precursor of type IV pilin in E. coli K-12 results in the production of pilin which comigrates with mature pilin, possibly because of processing by gspO (hopD)encoded prepilin peptidase [7, 12]. Another potential substrate for the GspO prepilin peptidase, PulG, is not cleaved when produced in E. coli K-12 [21, 23]. We inserted the region encompassing the gspA-gspC intergenic region into a promoter probe vector to test for the presence of potential promoters
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