Two NewSinorhizobium melilotiLysR-Type Transcriptional Regulators Required for Nodulation

Abstract

ABSTRACTThe establishment of an effective nitrogen-fixing symbiosis betweenSinorhizobium melilotiand its legume host alfalfa (Medicago sativa) depends on the timely expression of nodulation genes that are controlled by LysR-type regulators. Ninety putative genes coding for LysR-type transcriptional regulators were identified in the recently sequencedS. melilotigenome. All 90 putativelysRgenes were mutagenized using plasmid insertions as a first step toward determining their roles in symbiosis. Two new LysR-type symbiosis regulator genes,lsrAandlsrB, were identified in the screening. Both thelsrAandlsrBgenes are expressed in free-livingS. meliloticells, but they are not required for cell growth. AnlsrA1mutant was defective in symbiosis and elicited only white nodules that exhibited no nitrogenase activity. Cells of thelsrA1mutant were recovered from the white nodules, suggesting that thelsrA1mutant was blocked early in nodulation. AnlsrB1mutant was deficient in symbiosis and elicited a mixture of pink and white nodules on alfalfa plants. These plants exhibited lower overall nitrogenase activity than plants inoculated with the wild-type strain, which is consistent with the fact that most of the alfalfa plants inoculated with thelsrB1mutant were short and yellow. Cells of thelsrB1mutant were recovered from both pink and white nodules, suggesting thatlsrB1mutants could be blocked at multiple points during nodulation. The identification of two new LysR-type symbiosis transcriptional regulators provides two new avenues for understanding the complexS. meliloti-alfalfa interactions which occur during symbiosis.