The gene encoding the beta-1,4-endoglucanase (CelA) from Myxococcus xanthus: evidence for independent acquisition by horizontal transfer of binding and catalytic domains from actinomycetes

The celA gene encoding a beta-1,4 endoglucanase (CelA) from Myxococcus xanthus has been cloned in Escherichia coli and sequenced. The C-terminal region of CelA displayed a high level of similarity with the catalytic domain of several Egl belonging to the glycosyl hydrolases family 6 (CenA from Cellulomonas fimi, CelA from Microbispora bispora, E2 from Thermonospora fusca, CasA from Streptomyces KSM9 and CelA1 from Streptomyces halstedii) and less similarity to the cellobiohydrolases of the fungi Trichoderma reesei and Agaricus bisporus. Using PCR amplification we found in another myxobacterium, Stigmatella aurantiaca, a part of a glycosyl hydrolase belonging to the same family. The N-terminal part of CelA displayed significant similarities with the cellulose-binding domain of other cellulases belonging to a rare subset of family II, such as the avicelase I from Streptomyces reticuli, both tandem repeats N1 and N2 of the cellulase CenC from Cellulomonas fimi, and the N-terminal part of the Egl E1 from Thermonospora fusca. Analyses of the multiple alignments and reconstruction of phylogenetic trees strongly suggest that both domains of CelA were acquired by independent horizontal transfers between Gram+ soil bacteria and scavenging myxobacteria followed by domain shuffling.