Two types of pea leghemoglobin genes showing different O-2-binding affinities and distinct patterns of spatial expression in nodules

Five distinct cDNA clones for leghemoglobin (Lb) were isolated from pea (Pi sum sativum) nodules. They were classified into two groups designated PsLbA and PsLbB according to sequence homology, O-2-binding affinities of the re combinant proteins, and in situ localization of the mRNAs. The PsLbB group was comprised of four cDNA clones: PsLb120-1, -8, -29, and -34. They showed a high similarity of deduced amino acid sequences and O-2-binding affiniti es of their recombinant proteins. Among them, the spatial expression patter n of PsLb120-1 was investigated in great detail, indicating that its transc ripts were localized in the region from infection zone II to the distal par t of nitrogen fixation zone III in effective nodules. PsLb5-10, which is th e only cDNA clone of the PsLbA type, differed considerably from the PsLbB t ype in amino acid sequence, and the recombinant protein had a higher O-2-bi nding affinity than those of the PsLbB type. The transcripts of PsLb5-10 we re detected throughout the central tissue of effective nodules. However, in ineffective nodules on the pea mutant E135 (sym13), transcripts of PsLb5-1 0 were restricted to the distal portion of the central tissue as well as th ose of PsLb120-1. These findings indicate that the pea genome contains two types of Lb genes and suggest that they have different roles in the develop ment of nitrogen-fixing symbiosis in pea nodules.