Pseudomonas syringae Hrp type III secretion system and effector proteins

Pseudomonas syringae is a member of an important group of Gram-negative bacterial pathogens of plants and animals that depend on a type III secretion system to inject virulence effector proteins into host cells. In P. syringae, hrp/hrc genes encode the Hrp (type III secretion) system, and avirulence (avr) and Hrp-dependent outer protein (hop) genes encode effector proteins. The hrp/hrc genes of P. syringae pv syringae 61, P. syringae pv syringae B728a, and P. syringae pv tomato DC3000 are flanked by an exchangeable effector locus and a conserved effector locus in a tripartite mosaic Hrp pathogenicity island (Pai) that is linked to a tRNALeu gene found also in Pseudomonas aeruginosa but without linkage to Hrp system genes. Cosmid pHIR11 carries a portion of the strain 61 Hrp pathogenicity island that is sufficient to direct Escherichia coli and Pseudomonas fluorescens to inject HopPsyA into tobacco cells, thereby eliciting a hypersensitive response normally triggered only by plant pathogens. Large deletions in strain DC3000 revealed that the conserved effector locus is essential for pathogenicity but the exchangeable effector locus has only a minor role in growth in tomato. P. syringae secretes HopPsyA and AvrPto in culture in a Hrp-dependent manner at pH and temperature conditions associated with pathogenesis. AvrPto is also secreted by Yersinia enterocolitica. The secretion of AvrPto depends on the first 15 codons, which are also sufficient to direct the secretion of an Npt reporter from Y. enterocolitica, indicating that a universal targeting signal is recognized by the type III secretion systems of both plant and animal pathogens.

Phylogenetic relationships of the hoatzin, an enigmatic South American bird.

The hoatzin (Opisthocomus hoazin) lives in the humid lowlands of northern and central South America, often in riparian habitats. It is a slender bird approximately 65 cm in length, brownish with lighter streaks and buffy tips to the long tail feathers. The small head has a ragged, bristly crest of reddish-brown feathers, and the bare skin of the face is bright blue. It resembles a chachalaca (Ortalis, Cracidae) in size and shape, but its plumage and markings are similar to those of the smaller guira cuckoo (Guira guira). The hoatzin (pronounced Watson) has been a taxonomic puzzle since it was described in 1776. It usually has been viewed as related to the gallinaceous birds, but alliances to other groups have been suggested, including the cuckoos. We present DNA sequence evidence from the 12S and 16S rRNA mitochondrial genes, and from the nuclear gene that codes for the eye lens protein, alpha A-crystallin. The results indicate that the hoatzin is most closely related to the typical cuckoos and that the divergence occurred at or near the base of the cuculiform phylogenetic tree.

The hexapeptide LFPWMR in Hoxb-8 is required for cooperative DNA binding with Pbx1 and Pbx2 proteins.

The Hox gene products are DNA-binding proteins, containing a homeodomain, which function as a class of master control proteins establishing the body plan in organisms as diverse as Drosophila and vertebrates. Hox proteins have recently been shown to bind cooperatively to DNA with another class of homeodomain proteins that include extradenticle, Pbx1, and Pbx2. Hox gene products contain a highly conserved hexapeptide connected by a linker of variable length to the homeodomain. We show that the hexapeptide and the linker region are required for cooperativity with Pbx1 and Pbx2 proteins. Many of the conserved residues present in the Hoxb-8 hexapeptide are required to modulate the DNA binding of the Pbx proteins. Position of the hexapeptide relative to the homeodomain is important. Although deletions of two and four residues of the linker peptide still show cooperative DNA binding, removal of all six linker residues strongly reduces cooperativity. In addition, an insertion of 10 residues within the linker peptide significantly lowers cooperative DNA binding. These results show that the hexapeptide and the position of the hexapeptide relative to the homeodomain are important determinants to allow cooperative DNA binding involving Hox and Pbx gene products.