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A lasB-lacZ translational fusion (pTS400) was used to examine expression of the elastase gene (lasB) in Pseudomonas aeruginosa strain PAO1. Expression from the lasB-lacZ fusion was enhanced when PAO1(pTS400) was grown in a defined medium containing elevated levels of zinc (6.0 micrograms ml-1). Transcript accumulation studies on PAO1(pTS400) and PAO1 showed that the addition of zinc had a slight negative effect on lasB transcription. These results indicated that zinc regulates the expression of elastase at the translational level. A comparison between zinc regulation and iron regulation was also made. Iron has a negative effect on lasB-lacZ expression. When PAO1(pTS400) was grown in a defined medium with a low iron content (0.1 microgram ml-1) the bacteria still responded to zinc. The independent effects of low iron and high zinc concentrations suggest separate control mechanisms for the two factors. Transcript accumulation studies on PAO1 and PAO1 (pTS400) indicated that early in the growth curve iron did not influence transcription of lasB or lasB-lacZ. Later in the growth curve a slight increase in lasB-lacZ transcription was observed only in PAO1(pTS400) grown in low iron. These results suggest that the iron regulation of lasB occurs predominantly at the translational level. Finally, when PAO1(pTS400) was grown in a complex peptone-based medium, a high level of transcript accumulation accounted for elastase expression. Alterations of iron and zinc concentrations of this medium did not affect the expression of elastase. These results suggest that there may be additional environmental cues regulating lasB transcription.
Brumlik et al. (Sat,) studied this question.