AgfD, the checkpoint of multicellular and aggregative behaviour in Salmonella typhimurium regulates at least two independent pathways

The regulatory programme of multicellular behaviour in Salmonella typhimuri um is determined by mutations in the agfD promoter. AgfD has already been i dentified to regulate the extracellular matrix associated with the multicel lular morphotype composed of thin aggregative fimbriae (agf). To detect add itional components contributing to the multicellular morphotype in S. typhi murium, we constructed a mutant in agfD, the positive transcriptional regul ator of the agfBA(C) operon encoding for fimbrial subunit proteins. The agf D mutant lacked any form of multicellular behaviour as shown by analysis at the macroscopic and microscopic level. In contrast, the agfBA mutant unabl e to form thin aggregative fimbriae still maintained long-range intercellul ar adhesion. Promoter and expression analysis revealed that the genes downs tream of agfD agfEFG most likely did not contribute to the remaining aggreg ative behaviour. Screening of transcriptional fusions for agfD dependency u ncovered adrA, a homologue of yaiC in Escherichia coli. Environmental facto rs regulating adrA correspond to the regulation of thin aggregative fimbria e. AdrA is a putative transmembrane protein with a C-terminal GGDEF domain of unknown function although it is present in over 50 bacterial proteins. A drA mutant cells, which still formed thin aggregative fimbriae with all bin ding characteristics, exhibited community behaviour but, unlike the wild ty pe, lacked long-range intercellular adhesion. An agfBA adrA double mutant b ehaved like the agfD mutant. Therefore, it was concluded that agfD regulate s at least two independent pathways contributing to the multicellular morph otype in S. typhimurium.