Structure and conservation of a polyethylene glycol-degradative operon in sphingomonads

Institute for Bioresources, Okayama University, 2-20-1 Chuo, Kurashiki 710-0046, Okayama, Japan Correspondence Fusako Kawai fkawai{at}rib.okayama-u.ac.jp Sphingopyxis terrae , and Sphingopyxis macrogoltabida strains 103 and 203, can degrade polyethylene glycols (PEGs). They differ in the following respects: (i) different substrate specificities (chain length) of assimilable PEG, (ii) PEG-inducible or constitutive PEG-degradative proteins, and (iii) symbiotic or axenic degradation of PEG. S. terrae was able to incorporate PEG 6000, but strain 103 could not incorporate more than PEG 4000, suggesting that the difference in assimilable PEG chain length depends on the ability to take up substrate. PEG-degradative genes ( pegB , C , D , A , E and R ) from these strains were cloned. Their primary structures shared a high homology of more than 99 %. The peg genes encode a TonB-dependent receptor ( pegB ), a PEG-aldehyde dehydrogenase ( pegC ), a permease ( pegD ), a PEG dehydrogenase ( pegA ) and an acyl-CoA ligase ( pegE ), and in the opposite orientation, an AraC-type transcription regulator ( pegR ). The peg operon was flanked by two different sets of transposases. These three strains contained large plasmids and the operon was located in one of the large plasmids in S. terrae . The peg genes could be detected in other PEG-degrading sphingomonads. These results suggest that the peg genes have evolved in a plasmid-mediated manner. An insertion of a transposon gene ( pegF ) between pegD and pegA in strain 203 was found, which caused the constitutive expression of pegA in this strain. Abbreviations: PEG-DH, PEG dehydrogenase The GenBank/EMBL/DDBJ accession numbers for the sequences reported in this paper are AB196775 ( S. macrogoltabida strain 103), AB239080 ( S. macrogoltabida strain 203) and AB239603 ( S. terrae ). Details of the primers used in this study, the annotation of ORFs located within the peg gene cluster and its flanking region in S. macrogoltabida strain 103, the inducibility of PEG-DH in S. macrogoltabida strains 103 and 203, and Western blots using anti-PEG-DH antibody are available as supplementary data with the online version of this paper. These authors contributed equally to this paper.