An immunological comparison of nitrogenase proteins of fast and slow growing rhizobia

Abstract

The enzyme complex nitrogenase, which catalyzes nitrogen reduction consists of two component proteins [ 1,2]: component I, a molybdenum and iron containing protein comprising two types of subunits and component II, a dimer iron-sulfur protein with identical subunits. The actual nitrogen reduction takes place on component I and this component is called dinitrogenase; component II which supplies component I with electrons is called dinitrogenase reductase [3]. The nitrogenase complex shows a remarkable structural similarity [2] among widely divergent organisms. The genus Rhizobium can be divided into two groups, viz. slow and fast-growing species [4,5]. These two groups differ in several characteristics. The fastgrowing rhizobia have a generation time of 2-4 h, are acid producers, have laterally arranged flagella and do not reduce nitrogen in the free-living form [4,5]. The slow-growing rhizobia have a generation time of 6-8 h, are alkali producers, have polar or subpolar flagella and reduce nitrogen in free-living and symbiotic form. Furthermore there are differences in the metabolic pathways and DNA base ratios. The results of Ruvkun and Ausubel [6] on the sequence homology between the nifgenes of a fast and a slow-growing Rhizobium pointed to a rather restricted homology. This result implies that a substantial difference may exist between nitrogenase proteins of fast and slow-growing rhizobia. In this paper we report on the immunological comparison of component I proteins of fast and slowgrowing rhizobia by the use of radioimmunoassays. We have used this technique because comparative immunological analyses of proteins from different organisms have been useful in asserting taxonomic relationships [7,8] and only small amounts of proteins are required in this procedure.