Effect of cell density and mutation on the expression of Rhi genes in Rhizobium leguminosarum biovar viciae

Effects of cell density, symbiotic plasmid and mutation (by transposon mutagenesis) on the expression of rhi genes in Rhizobium leguminosarum biovar viciae were studied. Strains of the bacterium bearing and lacking regulatory gene rhiR were grown to late exponential phase and assayed for the production of rhi genes inducer. R. leguminosarum biovar viciae was specific for the synthesis of the rhi genes inducer. Expression of rhi genes advanced with increased cell population without any difference between Sym minus and Sym plus plasmid strains of the bacterium. Highest amount of the inducer was produced in the exponential phase. The growth of the microbe with and without Sym plasmid showed that symbiotic plasmid enhanced the optimal formation of rhi genes inducer. Iniducer formation in the absence of Sym plasmid was less than 20 Miller units of b-galactosidase activity in contrast to 2375 Miller units for plasmid-containing strain. Tn-5 mutagenesis generated four groups of mutants. Classes I, II, III and IV mutants were low, moderate-, highand super-producers of the inducer. These groups showed 145-250, 625-896, 1031-1375 and 1563 Miller units of phosphatase activity respectively.