Department of Microbiology, School of Medicine, Loma Linda University, CA 92350.
Genomic (chromosomal)hsd-Mu(lac) operon fusions have been constructed in two strains of Escherichia coli K-12 for the three hsd genes, hsdRK, hsdMK and hsdSK, using MudX and lambda placMu53. Expression of hsdK mutants ranged from 16 to 74 units (u) (with a mean of 52 u) for fusions to promoter pres and ranged from 26-75 u (also with a mean of 52 u) for fusions to promoter pmod. The expression of the two hsdK promoters was measured in different stages of growth. The pres fusion mutant showed a lag in beta-galactosidase (beta Gal) production, as compared to the pmod fusion mutant. One r-Km-K mutant (JR205) showed more than ten times the beta Gal activity of other insertion mutants. The activity of this mutant decreased by 20-fold upon the transfer of F101-102, which includes the wild-type hsd region. Positive gene-dosage effect was observed using F' plasmids containing the hsd-lacZ region.