Growth stage-dependent expression of 6-hydroxy-d-nicotine oxidase of the nicotine regulon of Arthrobacter oxidans

Gene structures and properties of enzymes of the plasmid-encoded nicotine catabolism of Arthrobacter nicotinovorans

Arthrobacter nicotinovorans is a Gram-positive aerobic soil bacterium able to grow on nicotine as its sole source of carbon and nitrogen. The initial steps of nicotine catabolism are catalyzed by nicotine dehydrogenase, the l- and d-specific 6-hydroxynicotine oxidases, and ketone dehydrogenase. The genes encoding these enzymes reside on a 160 kb plasmid, pAO1. The cccDNA of this plasmid was isolated in high purity and reasonable yield. It served as template material for the construction of a lambda-phage DNA library of the plasmid. The genes coding for 6-hydroxy-l-nicotine oxidase and for the subunits of the heterotrimeric ketone dehydrogenase were identified, subcloned and sequenced. The 6-hlno gene was identified as a 1278 bp open reading frame; its regulatory elements were also recognized. The derived primary structure of the monomer of apo-6-hydroxy-l-nicotine oxidase (46,264.5 Da) agrees with the data obtained by partial amino acid sequencing. 6-Hydroxy-l-nicotine oxidase and 6-hydroxy-d-nicotine oxidase were expressed in Escherichia coli and obtained in a state of high purity and crystallized. Ketone dehydrogenase (KDH) was found to be a heterotrimer with subunits of molecular mass 89,021.71, 26,778.65 and 17,638.88. The genes of KDH-A and KDH-B are juxtaposed; the A of the stop codon of KDH-A is used in the start codon of KDH-B, eliciting a frame shift. KDH-C is separated from KDH-A by 281 bp.

Interaction of the regulatory protein NicR1 with the promoter region of the pAO1-encoded 6-hydroxy-D-nicotine oxidase gene of Arthrobacter oxidans

The D,L-nicotine catabolism of the Gram-positive soil bacterium Arthrobacter oxidans is linked to the presence within the cells of the 160 kb catabolic plasmid pAO1. pAO1-cured cells lost the catabolic enzymes and reintroduction of pAO1 by electroporation into cured cells reestablished the nic+ phenotype. DNA band shift assays with extracts from cured and pAO1+ cells suggested that pAO1 encodes the regulatory protein NicR1. Footprint analysis revealed that two homologous palindromes (IR1 and IR2), present in the 5'-regulatory region of the 6-HDNO gene, were protected from DNase I digestion. Binding of NicR1 to the palindromes is symmetrical, co-operative, and stronger to IR1 containing the 6-HDNO gene promoter than to IR2. Site-directed mutagenesis revealed that steric constraints and sequence requirements for NicR1-binding are located exclusively in the palindromic sequences. Deletions and insertions in the interpalindromic region and in the 6-HDNO promoter -10 sequence had no effect on the binding characteristics of NicR1 to the 6-HDNO regulatory region. Acting as a repressor, NicR1 prevents binding of the E. coli RNA-polymerase to the consensus sigma 70 promoter in vitro. However, the interaction of NicR1 with the 6-HDNO promoter region in extracts of nicotine-induced cells from various growth stages did not differ from that observed with extracts of nicotine-uninduced cells.

Functional analysis of the 5' regulatory region and the UUG translation initiation codon of the Arthrobacter oxidans 6-hydroxy-D-nicotine oxidase gene

A functional analysis of the Arthrobacter oxidans 6-hydroxy-D-nicotine oxidase (6-HDNO) gene promoter (-35 region TTGACA and -10 region TATCAAT) and the UUG translation start codon was performed using site-directed mutagenesis. Deletion of the C residue from the -10 promoter region or mutations introduced upstream of the -10 region resulted in an increased 6-HDNO expression in Escherichia coli cells in vivo and in both E. coli and A. oxidans coupled transcription-translation systems in vitro. From the identical behaviour of 6-HDNO promoter mutants in the heterologous and homologous systems, it is concluded that A. oxidans harbours an RNA polymerase functionally homologous to the E. coli sigma 70 and Bacillus subtilis sigma 43 polymerases. Replacement of the TTG codon (UUG translation initiation codon) with ATG led to a 3.7-fold increase in 6-HDNO expression in E. coli. This effect was less pronounced at higher promoter strengths, from 3.7 in the case of the 6-HDNO wild-type promoter, to 2.5 in the case of the consensus -10 region and to 1.7 in the case of the tac promoter. A double point mutation introduced close to the ribosome binding site resulted in almost the same increase in 6-HDNO expression (3.1-fold) as the TTG-to-ATG exchange. The failure of cAMP to stimulate 6-HDNO expression in the A. oxidans system indicated that expression of this gene in stationary phase cells is not regulated by cAMP-catabolite repressore protein-mediated mechanism of catabolite repression.(ABSTRACT TRUNCATED AT 250 WORDS)