Expression of E. coli gal operon under the control of the prophage lambda promoter. PL leads to gross discoordinancy of gal expression. A survey of protein synthesized after prophage induction indicated that lack of expression of galE is due to a failure of translation of galE. Failure of translation of the galE sequence may be due to extensive dyad symmetry present in or near the gal promoter. This model was tested with insertions, deletions, or new promoter mutations within the symmetrical region. These strains restored some galE expression indicating that the RNA stem-loop structure plays a role in the discoordinate expression of gal from PL. Co-infection of mammalian cells with the E. coli gal-kinase operon and the herpes virus thymidine kinase genes has been used under selective conditions (for thymidine kinase) to obtain clones of cells which are transcribing and translating the E. coli gal-kinase gene. Drosophila melanogaster was surveyed for the enzymes of the Leloir pathway, no enzyme activity was detected, with the exception of trace amounts of 5-uridine diphosphogalactose-4-epimerase activity in the egg. This animal may be of use to study in vivo expression of the E. coli galactose genes.