The Pu and Pm promoters have many natural inducers that wander passively into gram-negative cells, and expression levels of cloned genes are affected by the type of inducer and its concentration. The vector copy-numbers can be set to any level between 5 and 100 per genome in the host cell, which is useful for numerous purposes. We have applied these vectors for metabolic engineering experiments in different bacterial species to control the biosynthesis of biopolymers such as xhanthan, alginate, and amylose. We are also using these vectors for industrial production of various human proteins in high-cell-denisty cultures of Escherichia coli. To further study this expression system, we are producing mutant banks for the -10 region and the XylS binding sites of Pm, and also for the untranslated leader mRNA region, and the xylS gene. The aim is to identify mutants with modified expression properties (e.g. elevated expression level, low uinduced expression level), and finally to obtain better control and understanding of this expression system.