The flagellar regulon presents a unique opportunity to study interoperon and global transcriptional regulation. Within the 14 operons in this regulon, there exists a variety of novel mechanisms that ensure the coordination of temporal expression and of transcription with the assembly of the flagellar structure itself. Additionally, it has been shown that one of the gene products of the "master" regulatory operon, FlhD, is a global regulator. This proposal will study how the FlhD/C heterotetramer regulates the flagellar regulon and how FlhD without FlhC regulates other operons. FlhD/C heterotetramer has a different DNA binding specificity than does FlhD by itself. The investigator will determine how the changes in the composition of these complexes alter DNA binding specificity. He will continue comprehensive structure/function analyses of FlhD and complexes containing FlhD. He will use allele-specific suppression analysis and X-ray crystallography to determine how FlhD interacts with FlhC and other possible complex partners. He will also characterize other nonflagellar DNA targets of FlhD and determine how the FlhD- containing complexes interact with these targets.