Fragments of Synechocystis-DNA driving salt-induced gene expression in E. coli were isolated with translational fusions to a 'lacZ gene. One fragment (fragment 19) showed a NaCl-dependent activation of beta Gal expression with the maximum of a ninefold increase in enzyme activity. A similar induction was triggered by the nonionic osmolyte sucrose, indicating an osmotically dependent activation. On the contrary, transcriptional activity of the DNA fragment 19 was only slightly enhanced under salt stress conditions, suggesting a posttranscriptional mechanism of induction. Primer extension assay was performed to identify the transcription initiation site. Upstream regions share weak homology to the "-10" hexamer consensus of E. coli sigma(70) promoters. The most thermodynamically stable secondary structure for the nontranslated part of the mRNA indicated that potential translation initiation sites might be blocked, leading to a low basal translation, whereas osmotic stress-induced changes of mRNA structure could be involved to increase translation. In order to analyze the function of fragment 19 in Synechocystis, promoter-probe plasmids were constructed allowing the stable integration of transcriptional and translational reporter gene fusions into the cyanobacterial chromosome. Quantitative assessment of reporter gene expression revealed a weak constitutive promoter activity of fragment 19 in Synechocystis. Sequence analysis showed that fragment 19 comprises 223 bp of the ORF s110747 of the Synechocystis genome.