The spliceosomal intron of the rolA gene of Agrobacterium rhizogenes is a prokaryotic promoter

Agrobacterium rhizogenes transfers DNA (T-DNA) from its Ri plasmid to plant cells. All T-DNA genes are expressed in plant cells. The rolA gene is the only T-DNA gene that contains an intron in the untranslated leader region o f its mRNA. This paper shows that (i) the rolA gene is also transcribed in bacteria; (ii) the 85 bp corresponding to the spliceosomal intron drives pr okaryotic gene expression in agrobacteria, in free-living rhizobia and in b acteroids within root nodules; and (iii) promoter activity is abolished by the deletion of 63 bp from its 5' end and is reduced by mutations changing its sequence near the putative -10 region. The expression pattern of a chim eric reporter gene shows that, in free-living bacteria, gene expression tak es place during the exponential phase of growth and increases at the onset of the stationary phase. Within root nodules, reporter gene expression occu rs in the invasion, nitrogen fixing and senescent zones.