Nucleotide Sequence and Genetic Analysis of the Region Essential for Functional Expression of the Gene for Ferredoxin I, fdxN, in Rhodobacter capsulatus: Sharing of One Upstream Activator Sequence in Opposite Directions by Two Operons Related to Nitrogen Fixation

Nucleotide sequencing of the region upstream of two ferredoxingenes, fdxC and fdxN, of Rhodobacter capsulatus revealed theexistence of one open reading frame (ORF), ORFU1,in the sameorientation as these genes and two other ORFs, ORFU2 and ORFU3,in the opposite orientation. Two potential –24/–12promoters were found in front of ORFU1 and ORFU2, respectively,and there was a putative upstream activator sequence (UAS) orNifA-binding site between them. The ORFs corresponded to noknown nif genes. However, analysis of their putative productsshowed that the product of ORFU1 (M_r 47,912) and that of ORFU3(M_r 19,090) flavodoxin-like domain and a 2[4Fe-4S] ferredoxin-likedomain, respectively, and that the product of ORFU2 (M_r 20,424)was a hydrophobic protein with six potential membrane-spanningportions. Results of interposon mutagenesis and complementationexperiments indicated ORFU2 but not ORFU1 is essential for nitrogenfixation and that additional gene(s) essential nitrogen fixationmust be present in the unsequenced region adjacent to ORFU3.Translational fusion analysis involving lacZYA and fdxN or ORFU3provided evidence that the putative UAS responsible for regulationof both ORFUl-fdxC-fdxNand ORFU2-ORFU3 operons in opposite orientations,and that the control of the latter is stricter than that ofthe former. (Received August 19, 1992; Accepted November 16, 1992)