Effect of oxygen level and presence of HupF on HupL status. Immunodetection of HupL and HypB proteins was carried out in crude cell extracts from R. leguminosarum cultures induced for hydrogenase activity under 1% O2 (A) or 3% O2 (B). Strains: UPM1155 derivative strains harboring plasmids pALPF1 (wt), pALPF2 (ΔhupL), pALPF14 (ΔhypC), and pALPF5 (ΔhupF). Proteins were resolved by SDS-PAGE in 9% (top panel) or 12% (bottom panel) acrylamide gels. Each lane was loaded with 60 μg (top panels) or 10 μg (bottom panels) of protein. Marks on the right margin indicate the location of the two forms of HupL protein: unprocessed HupL (u, 66 kDa), processed HupL (p, 65 kDa), or the position of molecular weight markers of the indicated size.