A cytochrome c biogenesis gene involved in pyoverdine production in Pseudomonas fluorescens ATCC 17400

Pseudomonas fluorescens ATCC 17400 produces pyoverdine under iron-limiting conditions. A Tn5 mutant, 2G11, produced lower amounts of different pyoverdine forms and was unable to grow under iron limitation caused by ethylenediamine-di(o-hydroxy-phenylacetic acid) (EDDHA) or zinc. This mutant was complemented by a 9.6 kb HindIII-BamHI DNA fragment that contained eight contiguous open reading frames (ORFs cytA to cytH). The proteins possibly encoded by this polycistronic gene cluster were all similar to the products of cytochrome c biogenesis genes from, amongst others, Rhodobacter capsulatus and Bradyrhizobium japonicum, not only in terms of amino acid sequence, but also in the overall hydropathy index of these proteins. By TnphoA mutagenesis and site-specific gene replacement it was found that the first three ORFs (cytA to cytC) were essential for cytochrome c production while only the product of cytA was needed for normal pyoverdine production. The presence of a putative haem-binding site in the CytA protein (WGSWWVWD) was confirmed. From analysis of a constructed phoA fusion, a periplasmic location was found for this motif. The ability of the cytA gene to restore both cytochrome c and pyoverdine production suggests the involvement of this particular gene both in haem and in pyoverdine transport in P. fluorescens.