Functional expression of Escherichia coli fhuA gene in Rhizobium spp. of Cajanus cajan provides growth advantage in pres
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O RI G I NAL PAPE R
Functional expression of Escherichia coli fhuA gene in Rhizobium spp. of Cajanus cajan provides growth advantage in presence of Fe3+: ferrichrome as iron source Geetha Rajendran · Shreni Mistry · Anjana J. Desai · G. Archana
Received: 7 August 2006 / Accepted: 20 October 2006 / Published online: 29 November 2006 © Springer-Verlag 2006
Abstract Cajanus cajan rhizobial isolates were found to be unable to utilize iron bound to ferrichrome, desferrioxamine B or rhodotorulic acid, all being hydroxamate type siderophores. A broad host range expression vector containing the Escherichia coli fhuA gene, encoding the outer membrane receptor for Fe-ferrichrome, was constructed. The plasmid construct (pGR1), designed to express fhuA under the lac promoter of E. coli, complemented E. coli MB97 fhuA mutant for ferri-ferrichrome utilization and also allowed Rhizobium spp. ST1 and Rhizobium spp. IC3123 to grow using iron bound to ferrichrome. Sensitivity to the antibiotic albomycin, transported via the FhuA receptor, was found in case of MB97 as well as rhizobial transformants harboring pGR1. The rhizobial transformants expressing fhuA showed growth stimulation when co-inoculated with Ustilago maydis, a fungal species known to produce ferrichrome under iron starved conditions. Growth stimulation was also observed in the presence of externally supplied ferrichrome. The signiWcance of these Wndings in terms of the potential for improving the survivability of rhizobial bioinoculant strains in natural soils is discussed. Keywords Rhizobium spp. · Ferrichrome uptake · E. coli fhuA gene · Heterologous gene expression · Cajanus cajan · Siderophore utilization
G. Rajendran · S. Mistry · A. J. Desai · G. Archana (&) Department of Microbiology and Biotechnology Centre, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, Gujarat, India e-mail: [email protected]
Introduction To meet iron requirements for growth, most microorganisms have developed high aYnity iron uptake systems for scavenging and transporting ferric iron (Braun and Killmann 1999). These involve low molecular weight compounds called siderophores which bind ferric iron with high aYnity (Neilands 1995; Crosa and Walsh 2002) and several membrane bound and periplasmic proteins that function together in the uptake of the ferric-siderophore complex (Faraldo-Gómez and Sansom 2003). More than 500 distinct siderophores have been reported to be secreted by microorganisms and their iron ligation groups have been classiWed into three main chemical types: hydroxamate, catecholate and hydroxyacid (Wandersman and Delepelaire 2004). Rhizobia are a group of gram-negative bacilli possessing the ability to form a nitrogen-Wxing symbiosis with members of the Leguminaceae family of plants. Recent taxonomy recognizes major groups of rhizobia as belonging to the Mesorhizobium–Sinorhizobium–Rhizobium group, the Bradyrhizobium group, the Azorhizobium group, Methylobacterium group, and Burkholderia group (Zakhia and deLajudi 2001; Sahgal and Johri
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