One or more transcriptional regulators of the LysR class control transcription of rhizobial nodulation genes. In Rhizobium sp. NGR234, two copies of nodD (nodD1 and nodD2) are present on the symbiotic plasmid pNGR234a. The complete sequence of pNGR234a revealed two additional nodD homologues, syrM1 and syrM2. Competitive RNA hybridization analyses involving a mutant of syrM1 (NGRΔsyrM1) showed that a number of symbiotic genes (e.g., nolXBTUVW) are expressed in an syrM1-dependent manner. Assays in which regions upstream of nolB and nolW were fused to promotorless lacZ confirmed that SyrM1 is required for their late induction. Mutation of syrM1 also drastically reduced production of sulfated Nod factors as shown by reverse phase-thin layer chromatography (RP-TLC). SyrM1 controls sulfation of Nod factors via one of the two chromosomal nodPQ loci. It thus seems likely that syrM1 of NGR234 encodes a transcriptional activator that regulates the expression of genes involved in both the early and late stages of infection.