The chromosomal arrangement of six soybean leghemoglobin genes.

Clones containing six leghemoglobin (Lb) genes have been isolated from two genomic libraries of soybean. They encompass two independent DNA regions: a 40-kb region containing four genes in the order 5' Lba-Lbc(1)-[unk]Lb-Lbc(3) 3' with the same transcriptional polarity, and another 40-kb region containing two genes in the order 5' Lbc(4)-Lbc(2) 3' with the same polarity. The order in which the Lb genes are arranged in the soybean genome imply that they are activated in the opposite order to which they are arranged on the chromosome. There is a close similarity between corresponding DNA regions outside the Lb genes in the two clusters. Thus, a moderately repetitive DNA element is present in corresponding positions in each cluster. In addition, at least two different non-Lb genes are linked to each Lb gene cluster in corresponding positions. These genes are apparently regulated in a way which differs from that of the Lb genes. The existence of two very similar Lb gene clusters in soybean suggest that soybean may have evolved from an ancestral form by genome duplication.