Sequencing and functional analysis of the nifENXorf1orf2 gene cluster of Herbaspirillum seropedicae

A 5.1-kb DNA fragment from the nifHDK region of H. seropedicae was isolated and sequenced. Sequence analysis showed the presence of nifENXorf1orf2 but nifTY were not present. No nif or consensus promoter was identified. Furth ermore, orf1 expression occurred only under nitrogen-fixing conditions and no promoter activity was detected between nifK and nifE, suggesting that th ese genes are expressed from the upstream nifH promoter and are parts of a unique nif operon. Mutagenesis studies indicate that nifN was essential for nitrogenase activity whereas nifX orf1 or f2 were not. High homology betwe en the C-terminal region of the NifX and NifB proteins from H. seropedicae was observed. Since the NifX and NifY proteins are important for FeMo cofac tor (FeMoco) synthesis, we propose that alternative proteins with similar a ctivities exist in H. seropedicae. (C) 1999 Federation of European Microbio logical Societies. Published by Elsevier Science B.V. All rights reserved.