Discovery of a rhizobial RNA that is essential for symbiotic root nodule development

All of the Azorhizobium, Bradyrhizobium, and Rhizobium genes known to be involved in the development of nitrogen-fixing legume root nodules are genes that code for proteins. Here we report the first exception to this rule: the sra gene; it was discovered during the genetic analysis of a Bradyrhizobium japonicum Tn5 mutant (strain 259) which had a severe deficiency in colonizing soybean nodules. A DNA region as small as 0.56 kb cloned from the parental wild type restored a wild-type phenotype in strain 259 by genetic complementation. The sra gene was located on this fragment, sequenced, and shown to be transcribed into a 213-nucleotide RNA. Results obtained with critical point mutations in the sra gene proved that the transcript was not translated into protein; rather, it appeared to function as an RNA molecule with a certain stem-and-loop secondary structure. We also detected an sra homolog in Rhizobium meliloti which, when cloned and transferred to B. japonicum mutant 259, fully restored symbiotic effectiveness in that strain. We propose several alternative functions for the sra gene product, of which that as a regulatory RNA for gene expression may be the most probable one.