The expression of lacZ has been analyzed and compared in a series of promoter cloning vectors by measuring the amount of lacZ mRNA by hybridization and the amount of -galactosidase by standard enzymatic assay. Expression was driven by the promoter, P_spc, of the spc ribosomal protein operon. The vectors contained either the standard W205 trp-lac fusion with the trp operon transcription terminator, trpt, located in the lacZ leader sequence, or a deletion derivative that functionally inactivates trpt. In the presence of trpt, lacZ expression was temperature dependent so that increasing the growth temperature reduced the accumulation of lacZ mRNA and -galactosidase activity. The frequency of transcript termination at trpt was estimated to be near zero at 20°C and at about 45% at 37°C. The amount of P_spc-derived lacZ mRNA and the amount of -galactosidase produced per lacZ mRNA varied, depending on the mRNA 5' leader sequence between P_spc and lacZ. These results demonstrate that the quantitative assessment of promoter activities with promoter cloning vectors requires careful analysis and interpretation. One particular construct without trpt did not seem to contain fortuitous transcription or translation signals generated at the fusion junction. In this strain, lacZ expression from P_spc was compared at the enzyme activity and mRNA levels with a previously constructed strain in which lacZ was linked to the tandem P1 and P2 promoters of the rrnB operon. At any given growth rate, the different activities of -galactosidase in these two strains were found to reflect the same differences in their amounts of lacZ mRNA. Assuming that the promoter-lacZ fusions in these strains reflect the properties of the promoters in their normal chromosomal setting, it was possible to estimate the absolute transcription activity of P_spc and the relative translation efficiency of P_spc-lacZ mRNA at different growth rates. Transcription from the spc promoter was found to increase from about 10 transcripts per min at a growth rate of 1.0 doublings/h to a maximum plateau of about 23 transcripts per min at growth rates above 1.5 doublings/h. The translation frequency of lacZ mRNA expressed from P_spc was unaffected by growth rates.