This study examined how the lactose transposon Tn951, carried on plasmid pGC9114, expresses β-galactosidase across different bacterial species. Researchers compared expression in Escherichia coli K12, Proteus mirabilis, Pseudomonas aeruginosa, and Pseudomonas putida. In E. coli, Tn951-encoded enzymes showed significantly lower induced activity levels and induction ratios compared to chromosomally-encoded lac genes, though expression varied slightly with strain background. In P. mirabilis and both Pseudomonas species, β-galactosidase expression was substantially more deficient—approximately 10-fold lower induced levels and drastically reduced induction ratios (2-5 compared to E. coli's higher values). The researchers compared Tn951 expression with F-factor-mediated lac expression where possible, confirming that anomalous expression in Proteus and Pseudomonas was not unique to the transposon. The authors discuss potential mechanisms for poor expression in non-E. coli hosts, including reduced RNA polymerase affinity for lac promoters, absence of cyclic AMP-CAP stimulation, lack of appropriate translation factors, or preferential mRNA degradation. The results demonstrate that gene expression from wide host-range plasmids varies significantly depending on the bacterial host, with important implications for genetic engineering and clinical microbiology.

Key findings

• Tn951-encoded β-galactosidase showed 1.5-7.7 fold lower maximum induced levels in E. coli compared to F-factor-mediated lac expression, with induction ratios reduced 7-52 fold
• In P. mirabilis, induced β-galactosidase levels were approximately 10-fold lower than in E. coli, with induction ratios reduced to 2-5
• Expression in Pseudomonas species was similarly deficient to P. mirabilis, with P. putida showing even more depressed enzyme levels
• E. coli strain background influenced lac expression parameters, including differences in induced activity levels and induction ratios across three K12 derivatives
• Anomalous lac expression in non-E. coli hosts likely involves multiple mechanisms: reduced promoter recognition, absent catabolite activator protein stimulation, translational deficiencies, or mRNA/protein degradation