ABSTRACT
Nitrogen assimilation is important during solvent production by Clostridium saccharobutylicum NCP262, as acetone and butanol yields are significantly affected by the nitrogen source supplied. Growth of this bacterium was dependent on the concentration of organic nitrogen supplied and the expression of the assimilatory enzymes, glutamine synthetase (GS) and glutamate synthase (GOGAT), was shown to be induced in nitrogen-limiting conditions. The regions flanking the gene encoding GS, glnA, were isolated from C. saccharobutylicum genomic DNA, and DNA sequencing revealed that the structural genes encoding the GS (glnA) and GOGAT (gltA and gltB) enzymes were clustered together with the nitR gene in the order glnA-nitR-gltAB. RNA analysis showed that the glnA-nitR and the gltAB genes were co-transcribed on 2.3 and 6.2 kb RNA transcripts respectively, and that all four genes were induced under the same nitrogen-limiting conditions. Complementation of an Escherichia coli gltD mutant, lacking a GOGAT small subunit, was achieved only when both the C. saccharobutylicum gltA and gltB genes were expressed together under anaerobic conditions. This is believed to be the first functional analysis of a gene cluster encoding the key enzymes of nitrogen assimilation, GS and GOGAT. A similar gene arrangement is seen in Clostridium beijerinckii NCIMB 8052, and based on the common regulatory features of the promoter regions upstream of the glnA operons in both species, we suggest a model for their co-ordinated regulation by an antitermination mechanism as well as antisense RNA.
Subject(s)
Bacterial Proteins/metabolism , Clostridium/metabolism , Gene Expression Regulation, Bacterial , Glutamate Synthase/metabolism , Glutamate-Ammonia Ligase/metabolism , Multigene Family , Nitrogen/metabolism , RNA-Binding Proteins/metabolism , Bacterial Proteins/chemistry , Bacterial Proteins/genetics , Base Sequence , Clostridium/enzymology , Clostridium/genetics , Clostridium/growth & development , Glutamate Synthase/chemistry , Glutamate Synthase/genetics , Glutamate-Ammonia Ligase/chemistry , Glutamate-Ammonia Ligase/genetics , Molecular Sequence Data , Nitrogen/pharmacology , RNA, Antisense/genetics , RNA, Antisense/metabolism , RNA-Binding Proteins/chemistry , RNA-Binding Proteins/geneticsABSTRACT
A full-length gene encoding a homologue of the small subunit of the glutamate synthase (GOGAT) enzyme was isolated from the anaerobic bacterium, Clostridium saccharobutylicum NCP262, which has been used extensively for the commercial production of solvents. Using a screening system designed to isolate genes involved in electron transport, plasmid pMET13C1 was isolated. Analysis of this plasmid identified a gene (1245 bp) with a predicted approximately 46-kDa product, which was associated with reductive activation of the pro-drug metronidazole. The deduced 414-amino-acid sequence was not typical of electron transport proteins, but rather shared striking homology to the small (beta) subunit of the GOGAT enzyme and other beta subunit-like polypeptides, and was thus designated gltX. Although all the functional domains typical of GOGAT beta subunits were conserved in this GltX protein, certain sequence features were not conserved. Furthermore, it was independently transcribed, did not lie adjacent to a GOGAT large subunit (alpha) domain, and its expression was not regulated by nitrogen conditions. These results provide additional support for current theories on the evolutionary relationships of GOGAT beta subunit domains in bacteria, and suggest that GltX belongs to a more general family of oxidoreductases, which is used in a context other than glutamate biosynthesis to transfer electrons to a currently unknown protein domain.
