Tj. Crease, SEQUENCE OF THE INTERGENIC SPACER BETWEEN THE 28S AND 18S RIBOSOMAL-RNA-ENCODING GENES OF THE CRUSTACEAN, DAPHNIA-PULEX, Gene, 134(2), 1993, pp. 245-249
The intergenic spacer (IGS) of the ribosomal DNA of the crustacean, Da
phnia pulex, is 4819 bp in length, and contains two non-repetitive reg
ions separated by two repetitive regions. The larger repetitive region
contains three complete and one truncated copy of a 330-bp sub-repeat
. Each sub-repeat contains the sequence, 5'-TATATAGGGAAG, which is ver
y similar to the promoter sequences found in repeated elements in the
IGS of other organisms. The 3' non-repetitive region, which separates
the repeat elements containing the putative transcription promoter sit
es from the 18S coding region, is large (2689 bp) compared to those de
scribed for other arthropods. A putative transcription start point (ts
p) has been identified 1280 bp upstream from the 18S coding region, su
ggesting that the external transcribed spacer is also large compared t
o that in other arthropods. The 3' non-repetitive region contains two
poly(GT) tracts of 18 and 20 bp. Such sequences, which are capable of
forming Z-DNA in vitro, have been found upstream from the transcriptio
n units of rRNA and protein-coding genes in human, mouse and rat, and
are thought to have a functional role in gene expression.