The secreted endoglucanase (CenA) from the Gram-positive bacteriumCellulomonas fimiand a deletion derivative (ΔCenA) lacking the N-terminal leader peptide of native CenA were used to explore the potential of ΔCenA as a reporter molecule inCaulobacter crescentus. Expression ofcenAinC.crescentusyielded extracellular endoglucanase activity, suggesting that the N-terminal leader peptide of CenA could direct the enzyme to the periplasm where it subsequently leaked into the medium. In contrast, expression of ΔcenAyielded only cell-associated endoglucanase activity; this suggested that the enzyme retained activity in theC.crescentuscytoplasm. Using the putative cytoplasmic and periplasmic forms of ΔCenA as markers, a simple assay for periplasmic ΔCenA hybrids was developed. This assay indicated that ΔCenA activity was largely independent of cellular location. To facilitate the use of ΔCenA as a reporter, a broad host range translational fusion vector (pEC215) incorporating ΔcenAwas constructed. This vector was used to investigate factors important to the expression of the gene (rsaA) encoding the paracrystalline surface protein (S-layer) of the bacterium. It was found that altering the 5′ untranslated region of thersaAmRNA reduced gene expression by 70%. OnersaA:ΔcenAgene fusion resulting from these experiments that incorporated onlyrsaAtranslation initiation information was further modified to serve as a general reporter for creating transcriptional gene fusions with other promoters. Gene fusions between alkaline phosphatase (phoA) and eithercenAorlacZwere used to supplement information about RsaA secretion derived fromrsaA:phoAgene fusions. It was found that linkage of the N-terminal leader peptide of CenA to PhoA yielded 50–100 times more cell-associated PhoA activity inC.crescentusthan linkage of the RsaA N terminus. Taken together, these experiments indicated that ΔCenA was useful for tagging proteins localized to the cytoplasm, exported to the periplasm, or secreted from the cell, as well as for monitoring events in the cytoplasm such as examining factors important to the level of gene expression. Further, because ΔCenA was active in all cell compartments, it could be used to estimate the efficiency of hybrid protein export-secretion from enzyme activity measurements alone. In short, ΔCenA possessed many of the attributes of an "all-purpose" reporter.Key words: gene fusions, protein secretion, cellulase, alkaline phosphatase,Caulobacter crescentus.