Quorum sensing in soft-rotting Erwinia carotovora
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The soft-rotting E. carotovora subspecies (ssp.) produces effectors and an array ofextra cellular enzymes, including pectate lyase (Pel), polygalacturonase (Peh), cellulase (Cell) and protease (Prt) that are known or predicted to function as virulence and pathogenicity factors. The production of these exoproteins is activated by quorum sensing (QS)signal, N-acyl homoserine lactone (AHL). At elevated temperatures, however, a majority of E. c. ssp. atroseptica (Eca), betavasculorum (Ecb) and carotovora (Ecc) strains produce much reduced amounts of AHL as well as those exoproteins, and activate the production of a global negative regulator, RsmA (Rsm = regulator of secondarymetabolites). Ecc strain EC153 is an exception in that it produces higher levels of exproteins as well as AHL at 34.5[degree sign]C than at 28[degree sign]C. Temperature-dependent production of virrulence factors by these strains correlates with overall metabolic activities and stabilities of ahlI (the gene for AHL synthase), pel-1 and peh-1 transcripts. EC153 also causes extensive maceration of celery and Chinese cabbage petioles at 34.5[degree sign]C, which contrasts with limited tissue maceration by Ecc strain Ecc71 at this temperature. Thus, overall metabolic activity, higher levels of AHL and greater mRNA stability of virulence activates transcription of rsmA. This activation of rsmA is prevented by AHL. Consequently, in the presence of AHL RsmA production occurs at a low level, triggering the production of virulence factors. These findings for the first time document regulation of an RNA-binding protien by ExpR and AHL, link quorum sensing system via posttransciptional regulation, and explain the basis for the pleiotropic effects of AHL in Ecc. This study also reports the existence of two classes of strains based on structural and functional characteristics of ExpR and AhlI in Erwinia carotovora subspecies. While class I strains produce 3-oxooctanoyl-L-homoserine lactone (3-oxo-C8-HL) as the major AHL analog and 3-oxohexanoyl homoserine lactone (3-oxo-C6-HL) as a minor component, the members of class II strains produce 3-oxo-C6-HL as the major analog. It has also been found that AHL synthase is responsible for biosynthetic specificity. ExpR species do not discriminate between rsmA genes of strains belonging to the two classes. However, specificity is conferred by AHL analogs in that 3-oxo-C8-HL neutralizes the actions of class II ExpRs whereas 3-oxo-C6HL neutalizes class I ExpRs.
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