The relative degrees of repression of the ydhY -- lacZ fusion in the presence of nitrite ( approximately twofold ) or nitrate ( ~ 13-fold ) suggest that NarL-mediated inhibition of FNR recruitment is a key component of the regulation of the ydhY -- T promoter , and that NarP-mediated inhibition of RNA polymerase binding is partially overcome by the ability of FNR to recruit RNA polymerase .