Ramakrishnan, T. (Yale University, New Haven, Conn.), and Edward A. Adelberg. Regulatory mechanisms in the biosynthesis of isoleucine and valine. III. Map order of the structural genes and operator genes. J. Bacteriol. 89:661-664. 1965.-A new method has been employed to determine the map order of the structural genes and operator genes governing the enzymes of the isoleucine-valine biosynthetic pathway. This method relies on the observation that phage transduction of markers carried on an F-genote leads to the establishment in the recipient of F-genotes of various lengths. Using this method, we have established that the order of loci is the following: F/ilvE ilvD ilvA oprA/ilvC/ilvB oprB. The operator locus, oprA, regulates the activity of structural genes ilvE (transaminase B), ilvD (dehydrase), and ilvA (threonine deaminase). The operator locus, oprB, regulates the activity of ilvB (condensing enzyme). An operator for ilvC (reductoisomerase) can be inferred to exist, but has not yet been detected genetically. The loci ilvB and oprB have been shown to be at the extreme right end of the sequence, but their positions relative to each other remain to be established.