Determination and Analysis of the Putative AcaCD-Responsive Promoters of Salmonella Genomic Island 1

The integrative genomic island SGI1 and its variants confer multidrug resistance in numerous Salmonella enterica serovariants and several Proteus mirabilis and Acinetobacter strains. SGI1 is mobilized by the IncA/C family plasmids. The island exploits not only the conjugation apparatus of the plasmid, but also utilizes the plasmid-encoded master regulator AcaCD to induce the excision and formation of its transfer-competent form, which is a key step in the horizontal transfer of SGI1. Triggering of SGI1 excision occurs via the AcaCD-dependent activation of xis gene expression. AcaCD binds in P(xis) to an unusually long recognition sequence. Beside the P(xis) promoter, upstream regions of four additional SGI1 genes, S004, S005, S012 and S018, also contain putative AcaCD-binding sites. Furthermore, SGI1 also encodes an AcaCD-related activator, FlhDC(SGI1), which has no known function. Here, we have analysed the functionality of the putative AcaCD-dependent promoter regions and proved their activation by either AcaCD or FlhDC(SGI1). Moreover, we provide evidence that both activators act on the same binding site in P(xis) and that FlhDC(SGI1) is able to complement the acaCD deletion of the IncA/C family plasmid R16a. We determined the transcription start sites for the AcaCD-responsive promoters and showed that orf S004 is expressed probably from a different start codon than predicted earlier. Additionally, expression of S003 from promoter P(S004) was ruled out. P(xis) and the four SGI1 promoters examined here also lack obvious -35 promoter box and their promoter profile is consistent with the class II-type activation pathway. Although the role of the four additionally analysed AcaCD/FlhDC(SGI1)-controlled genes in transfer and/or maintenance of SGI1 is not yet clear, the conservation of the whole region suggests the existence of some selection for their functionality.