The hrp gene locus of Pseudomonas solanacearum, which controls the production of a type III secretion system, encodes eight proteins related to components of the bacterial flagellar biogenesis complex

Abstract

Five transcription units of the Pseudomonas solanacearum hrp gene cluster are required for the secretion of the HR-inducing PopA1 protein. The nucleotide sequences of two of these, units 1 and 3, have been reported. Here, we present the nucleotide sequence of the three other transcription units, units 2, 4 and 7, which are together predicted to code for 15 hrp genes. This brings the total number of Hrp proteins encoded by these five transcription units to 20, including HrpB, the positive regulatory protein, and HpaP, which is apparently not required for plant interactions. Among the 18 other proteins, eight belong to protein families regrouping proteins involved in type III secretion pathways in animal and plant bacterial pathogens and in flagellum biogenesis, while two are related solely to proteins involved in secretion systems. For the various proteins found to be related to P. solanacearum Hrp proteins, those in plant-pathogenic bacteria include proteins encoded by hrp genes. For Hrp-related proteins of animal pathogens, those encoded by the spa and mxi genes of Shigella flexneri and of Salmonella typhimurium and by the ysc genes of Yersinia are involved in type III secretion pathways. Proteins involved in flagellum biogenesis, which are related to Hrp proteins of P. solancearum, include proteins encoded by fli and flh genes of S. typhimurium, Bacillus subtilis and Escherichia coli and by mop genes of Erwinia carotovora. P. solanacearum Hrp proteins were also found to be related to proteins of Rhizobium fredii involved in nodulation specificity.

The FliP and FliR proteins of Salmonella typhimurium, putative components of the type III flagellar export apparatus, are located in the flagellar basal body

“…FliP and FliR (and several other flagellar proteins) have homologues in type III systems responsible for the export of virulence factors: SpaP and SpaR in Salmonella (Groisman and Ochman, 1993), Spa24 and Spa29 in Shigella flexneri (Sasakawa et al, 1993), YscR and YscT in Yersinia (Bergman et al, 1994) and HrpT and HrpC in Pseudomonas solanacearum (Van Gijsegem et al, 1995). This makes it virtually certain that these homologous components are organized and function similarly in the two systems.…”

“…This export process uses a type III secretory pathway, which is signal peptide independent and involves many protein components. Although type III secretory pathways have been identified in a variety of animal and plant pathogenic bacteria Van Gijsegem et al, 1995;Galá n, 1996), little is known about the mechanism of export or the function of each of the individual proteins involved. The same is true for the pathway for flagellar protein export.…”

Hrp pilus: An hrp -dependent bacterial surface appendage produced by Pseudomonas syringae pv. tomato DC3000

“…Sixteen of the 25 genes in this completely sequenced hrp͞hrmA gene cluster are either predicted or shown to be required for secretion of harpin Pss , a proteinaceous elicitor of the HR encoded by hrpZ (9,10). Nine of these hrp genes, recently renamed hrc genes (11), are broadly conserved among P. syringae pathovars, Erwinia, Xanthomonas, and Ralstonia (9,(12)(13)(14)(15). hrc genes, including hrcC (formerly hrpH; ref.…”

“…Sixteen of the 25 genes in this completely sequenced hrp͞hrmA gene cluster are either predicted or shown to be required for secretion of harpin Pss , a proteinaceous elicitor of the HR encoded by hrpZ (9, 10). Nine of these hrp genes, recently renamed hrc genes (11), are broadly conserved among P. syringae pathovars, Erwinia, Xanthomonas, and Ralstonia (9,[12][13][14][15] (20), and inv genes of Salmonella (21, 22), all of which function in secretion of proteins required for pathogenesis. The protein products of hrc genes and related counterparts in animal pathogenic bacteria are predicted to be components of a novel protein secretion pathway, the so-called type III secretion pathway, in Gram-negative bacteria.…”

The type III secretion injectisome, a complex nanomachine for intracellular ‘toxin’ delivery

“…Among the nine conserved proteins, eight are shared with the flagellum (Fields et al, 1994;Woestyn et al, 1994;Van Gijsegem et al, 1995), suggesting a common evolutionary origin, which is also evidenced by the similarity of their basal body structure (Kubori et al, 1998). Injectisomes have evolved into seven different families (Gophna et al, 2003;Pallen et al, 2005;Troisfontaines and Cornelis, 2005).…”