GENES FOR D-ARABINITOL AND RIBITOL CATABOLISM FROM KLEBSIELLA-PNEUMONIAE

The enzymes for catabolism of the pentitols D-arabinitol (Dal) and rib itol (Rbt) and the corresponding genes from Klebsiella pneumoniae (dal and rbt) and Escherichia coli (atl and rtl) have been used intensivel y in experimental evolutionary studies. Four dal and four rbt genes fr om the chromosome of K. pneumoniae 1033-5P14 were cloned and sequenced . These genes are clustered in two adjacent but divergently transcribe d operons and separated by two convergently transcribed repressor gene s, dalR and rbtR. Each operon encodes an NAD-dependent pentose dehydro genase (dalD and rbtD), an ATP-dependent pentulose kinase (dalK and rb tK) and a pentose-specific ion symporter (dalT and rbtT). Although the biochemical reactions which they catalyse are highly similar, the enz ymes showed interesting deviations. Thus, DalR (313 aa) and RbtR (270 aa) belong to different repressor families, and DalD (455 aa) and RbtD (248 aa), which are active as a monomer or as tetramers, respectively , belong to different dehydrogenase families. Of the two kinases (19.3 % identity), DalK (487 aa) belongs to the subfamily of short D-xylulok inases and RbtK (D-ribulokinase; 535 aa) to the subfamily of long kina ses. The repressor, dehydrogenase and kinase genes did not show extens ive similarity beyond local motifs, This contrasts with the ion sympor ters (86.6% identity) and their genes (82.7% identity). Due to their u nusually high similarity, parts of dalT and rbtT have previously been claimed erroneously to correspond to 'inverted repeats' and possible r emnants of a 'metabolic transposon' comprising the dal and rbt genes. Other characteristic structures, e.g. a secondary att lambda site and chi-like sites, as well as the conservation of this gene group in E. c oil C are also discussed.