Activity of two different silencer elements of the chicken lysozyme gene can be compensated by enhancer elements

The chicken lysozyme gene is constitutively expressed in macrophages. Transfection of recombinant genes containing different portions of the lysozyme 5' upstream region revealed the existence of two negative transcriptional elements within 1 kb upstream of the start sites. Both elements placed upstream or downstream of a heterologous promoter-gene unit repress transcription independent of their orientation and are therefore called silencer elements, although their repressing activities 3' of the gene are reduced. One silencer (N-1.0 kb) at position -1 kb consists of the central region of the chicken middle repetitive sequence element CR1 and can be divided into two functional domains. N-1.0 kb is active in all cell types tested. The other silencer (N-0.25 kb) at position -0.25 kb shows reduced activity in primary macrophages. Despite their different specificities, the activity of both silencer elements can be influenced similarly. An inverse linear relationship between the transcriptional activity of the tested constructs and the potential inhibition by the silencer elements was found: weak transcription units can be strongly repressed, whereas strong transcription units can be only weakly repressed. Such a mechanism may help to turn off completely a particular gene in situations or tissues where strong positive regulators are inactive.