Diversity of the ring-cleaving dioxygenase gene pcaH in a salt marsh bacterial community

Degradation of lignin-related aromatic compounds is an important ecological process in the highly productive salt marshes of the southeastern United States, yet little is known about the mediating organisms or their catabolic pathways. Here we report the diversity of a gene encoding a key ring-cleaving enzyme of the beta-ketoadipate pathway, pcaH, amplified from bacterial communities associated with decaying Spartina alterniflora, the salt marsh grass that dominates these coastal systems, as well as from enrichment cultures with aromatic substrates (p-hydroxybenzoate, anthranilate, vanillate, and dehydroabietate). Sequence analysis of 149 pcaH clones revealed 85 unique sequences. Thirteen of the 53 amino acid residues compared were invariant in the PcaH proteins, suggesting that these residues have a required catalytic or structural function. Fifty-eight percent of the clones matched sequences amplified from a collection of 36 bacterial isolates obtained from seawater, marine sediments, or senescent Spartina. Fifty-two percent of the pcaH clones could be assigned to the roseobacter group, a marine lineage of the class alpha-Proteobacteria abundant in coastal ecosystems. Another 6% of the clones matched genes retrieved from isolates belonging to the genera Acinetobacter, Bacillus, and Stappia, and 42% of the clones could not be assigned to a cultured bacterium based on sequence identity. These results suggest that the diversity of the genes encoding a single step in aromatic compound degradation in the coastal marsh examined is high.