Abstract
Chromosomes of higher eukaryotes are thought to be organized into a
series of discrete and topologically independent higher-order domains. In
addition to providing a mechanism for chromatin compaction, these
higher-order domains are thought to define independent units of gene
activity. Implicit in most models for the folding of the chromatin fiber are
special nucleoprotein structures, the domain boundaries, which serve to
delimit each higher-order chromosomal domain. We have used an
"enhancer-blocking assay" to test putative domain boundaries for boundary
function in vivo. This assay is based on the notion that in delimiting
independent units of gene activity, domain boundaries should be able to
restrict the scope of activity of enhancer elements to genes which reside
within the same domain. In this case, interposing a boundary between an
enhancer and a promoter should block the action of the enhancer. In the
experiments reported here, we have used the yolk protein-1 enhancer element
and an hsp70 promoter:lacZ fusion gene to test putative boundary DNA
segments for enhancer-blocking activity. We have found that several scs-like
elements are capable of blocking the action of the yp-1 enhancer when placed
between it and the hsp70 promoter. In contrast, a MAR/SAR DNA segment and
another spacer DNA segment had no apparent effect on enhancer
activity.