The regulatory gene, glnF, of Escherichia coli was fused to the structural genes of the lac operon by use of the hybrid Mu phage derivative Mudl (Ap lac). Analysis of two of these fusions showed that the glnF gene is expressed constitutively, i.e., independent of either the nitrogen source in the growth medium or the availability of the glnA, glnL, glnG or glnF functional gene products. The orientation of the Mud1 (Ap lac) insertions was determined by chromosome mobilization in F-merogenotes carrying either of the two glnF::Mud1 chromosomal insertions isolated, and either one of a pair of F'lacZ::Mucts62 episomes; the two episomes differing in that their Mucts62 insertions are located in opposite orientations with regard to lacZ. The direction of chromosome mobilization by the Hfrs that were probably formed via Mu homology demonstrated that orientation of the glnF gene is clockwise relative to that of the chromosome.