2% agarose gels, and nifH bands were excised and then cleaned using a QIAquick Nucleotide Removal Kit (Qiagen). Cleaned fragments were inserted into #links# a pCR?2.1-TOPO? vector using One Shot? TOP10 Chemically Competent E.?coli, TOPO TA Cloning? Kit (Invitrogen) according to manufacturer's specification. Clones were picked randomly and amplified using M13 forward (?20) and M13 reverse primers.