What is stringent vector? - Is it related to level of expression? (Jun/11/2009 )

What is stringent vector? Is the term 'stringent' related to level of expression? What are other types of expression vectors?
What are the examples? Is there any reading material available on this? I googled, but could not find proper information.
Thanks
Ram

Assume you express your protein in E coli.
There are many vectors, some use T7 promoter, some lac, T7/lac etc... Each promoter has different stringency. T7 is known to be leaky, so some vector will include a lacI to prevent the leakage, some will have pLysS etc.
try http://www.bio.net/bionet/mm/methods/1999-March/074525.html

Thank you for the information.
Yes. Its for expression in E coli
So the adjective 'stringent' before vector is always related to the promoter of the vector..right?
As you said, expression systems consisting of T7 promoter are considered to be leaky, meaning they are not stringent. Why pET vectors (containing T7 promoter) are considered stringent? Can you comment anything on 'stringency' of tet promoter systems (eg. pASK-IBA7) ? If I am getting very low level of expression with pASK-IBA7 vector, does it mean that I may get good levels in pET since this is not a stringent vector (or is it?) CONFUSED! What are other non-stringent vectors?
The link that u posted is not opening