The
corresponding physical locus was defined as the DNase I-sensitive
domain. This was elucidated by Susan Wykes. The results of these
studies were published in the Journal
of Biological Chemistry (1995) 270: 8755-8762.
The
protamine genic domain resides within a single DNase I-sensitive
region. The DNase I-sensitivity profile for each data point can
be displayed by clicking on the picture. The open diamonds indicate
potential regions of matrix association, or MARs. These were primarily
identified by a computer algorithm (Kramer
and Krawetz, 1995),
and then comfirmed by a biochemical assay (Kramer and Krawetz, 1996).
This domain of DNase I sensitivity remains constant throughout spermatogenesis
(Kramer et al., 2000)
Highly
condensed regions of chromatin (i.e. heterochromatin) are transcriptionally
silent as the DNA is not accessible to the "transcription machinery"
(i.e. transcription and transcription associtated factors of the
preinitiation complex). DNase I sensitivity, which measures the
accessiblity (to nuclease activity) of DNA, provides an assay to
measure how, at any one time, regions of chromatin are accessible
(ie. potentiated). The figure below outlines the
steps necessary to promote transcription. The domain must be Potentiated
so that the preinitiation complex can associate with elements at
the promoter and enahncer. Once this is accomplished, RNA polymerase
(shown in red) Initiates transcription. Once the
RNA polymerase has cleared the promoter, Elongation
of the RNA transcript proceeds.
Transgenic
and DNase I-sensitivity analysis has led to the following model
of gene selection in the haploid spermatid. (Kramer et al.
1998).
The
members of the PRM1>PRM2>TNP2 locus are first potentiated
in the late pachytene spermatocyte, transcribed in round spermatids,
and their products translated at the elongating spermatid stage
of differentiation. The Phosphoglycerate kinase genes (Pgk1 and
Pgk2) are another example of genes that are differentiatlly regulated
by X-inactivation and potentiation throughtout spermatogenesis.
When Pgk1 (on the X chromsome) becomes inactivated during meiosis,
Pgk2 (an autosomal, testes specific form) is potentiated and transcribed.
DNase I hypersensitive sites (red stars) are marked. These regions
are commonly related with regulatory regions.
