Tightest promoter repression ?

[the End]

I need to find a promoter which is very tigthly regulated for use in reducing
the expression of downstream Lac sequence to a very low level.

I currently am using the tac promoter with the lac IQ allele carried at pUC
copy number in a second plasmid. Even with this repressor level I am seeing a
certain amount of background lac Z expression in the absence of inducer.

Tell me your experiences in using regulated promoters. I prefer to use a 
rather strong single promoter (eg. not Pgal) that will function well
in vitro.

Thanks much,


Jim