Analysis of 4-Phosphopantetheinylation of Polyhydroxybutyrate Synthase from Ralstonia eutropha: Generation of β-Alanine Auxotrophic Tn5 Mutants and Cloning of the panD Gene Region

The postulated posttranslational modification of the polyhydroxybutyrate (PHA) synthase from Ralstonia eutropha by 4-phosphopantetheine was investigated. Four β-alanine auxotrophic Tn5-induced mutants of R. eutropha HF39 were isolated, and two insertions were mapped in an open reading frame with strong similarity to the panD gene from Escherichia coli, encoding l-aspartate-1-decarboxylase (EC 4.1.1.15), whereas two other insertions were mapped in an open reading frame (ORF) with strong similarity to the NAD(P)(+) transhydrogenase (EC 1.6.1.1) alpha 1 subunit, encoded by the pntAA gene from Escherichia coli. The panD gene was cloned by complementation of the panD mutant of R. eutropha Q20. DNA sequencing of the panD gene region (3,312 bp) revealed an ORF of 365 bp, encoding a protein with 63 and 67% amino acid sequence similarity to PanD from E. coli and Bacillus subtilis, respectively. Subcloning of only this ORF into vectors pBBR1MCS-3 and pBluescript KS(−) led to complementation of the panD mutants of R. eutropha and E. coli SJ16, respectively. panD-encoded l-aspartate-1-decarboxylase was further confirmed by an enzymatic assay. Upstream of panD, an ORF with strong similarity to pntAA from E. coli, encoding NAD(P)(+) transhydrogenase subunit alpha 1 was found; downstream of panD, two ORFs with strong similarity to pntAB and pntB, encoding subunits alpha 2 and beta of the NAD(P)(+) transhydrogenase, respectively, were identified. Thus, a hitherto undetermined organization of pan and pnt genes was found in R. eutropha. Labeling experiments using one of the R. eutropha panD mutants and [2-(14)C]β-alanine provided no evidence that R. eutropha PHA synthase is covalently modified by posttranslational attachment of 4-phosphopantetheine, nor did the E. coli panD mutant exhibit detectable labeling of functional PHA synthase from R. eutropha.