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The P promoter allows for tight regulation and control of a target gene in vivo.  As explained above, P is regulated by the addition and absence of arabinose. As tested, the promoter can be further repressed with reduced levels of cAMP through the addition of glucose. Plasmid vectors have been constructed and tested with a selectable marker (Cm in this case), origin of replication, araC and operons, multiple cloning site and P promoter. Studies show that vectors are highly expressed and can be used, in combination with chromosomal null alleles, to study loss of function of essential genes.