The expression of the avian clusterin gene can be driven by two alternative promoters with distinct regulatory elements

Clusterin cDNA has been isolated as a copy of a mRNA overexpressed in a wide variety of biological disorders, including tissue regression, brain injuries and oncogenic cell transformation. While the molecular cloning of the rat and the human clusterin genes has revealed a high degree of conservation of the genomic organization between mammals, the avian locus described here illustrates several divergent features. The avian gene has the particularity to be transcribed from at least two different promoters, both of which are active in transient expression assays using the quail QT6 transformed cell line. The detection of the two clusterin mRNA species by reverse-transcription-mediated PCR reveals a coordinated initiation of transcription from both promoters in all organs tested. In possible relation to the bipartite organization of the avian regulatory region, the putative cis-elements described in the unique mammalian promoters appear divided among the two avian promoters. In addition, the sequence comparison of avian and mammalian regulatory sequences has allowed the identification of a conserved putative cis-element which appears to be the target for specific DNA-binding factors.

RNA-dependent DNA binding activity of the Pur factor, potentially involved in DNA replication and gene transcription

The PUR element is a polypurine polypyrimidine motif that can stimulate transcription, encountered in the 5' regions of various genes and in the vicinity of several DNA replication initiation zones. We demonstrate that the PUR complex formation between the purine-rich strand of PUR and nuclear extracts can be prevented by pretreatment of nuclear extracts with RNA-damaging agents such as UV light or RNase A. A biochemical affinity method reveals that small RNA molecules copurify with the Pur factor. Moreover, the PUR binding activity of RNA-depleted nuclear extracts can be restored by addition of phenol-extracted RNAs. This work adds a new member in the emerging class of ribonucleoprotein particles as regulatory factors of the genetic expression.

Serum factors and v-src control two complementary mitogenic pathways in quail neuroretinal cells in culture

Quail neuroretinal cells (QNR cells) from 7-day-old embryos do not proliferate even in the presence of 8% fetal calf serum. After infection by the Rous sarcoma virus (RSV) they proliferate actively and exhibit a transformed phenotype; this effect is mediated by the oncoprotein pp60v-src. Secondary cultures infected by the thermosensitive strain tsNY68 of RSV are blocked in G0 either by thermal inactivation of pp60v-src at 41.5 degrees C or by serum deprivation at the permissive temperature (36.5 degrees C). Cell division is reinduced either by pp60v-src thermal renaturation or by subsequent serum addition. Our results indicate that v-src and serum control two synergic pathways leading to G0/G1 transition in QNR cells. In order to characterize genes related to the mitogenic and transforming effects of v-src in nerve cells, we have constructed a cDNA library from QNR cells transformed by tsNY68. We report the properties of five molecular clones isolated by differential screening of this library. Unlike immediate-early genes like c-fos, they are induced in mid and late G1. Four of them correspond to unknown mRNAs and the last one codes for nucleolin. This set of v-src-regulated genes is likely to code for functions deficient in terminally differentiated QNR cells and necessary for the progression in G1.

V-src-induced-transcription of the avian clusterin gene

We have isolated the avian gene T64 corresponding to the mammalian clusterin, on the basis of high accumulation of its template mRNA in cells infected with oncogenic retroviruses. Since the clusterin was shown to have a protective effect against the immune system, its induction by oncogenic viruses is of major biological importance. The unique, short 5 kb-long T64 genomic locus is inactive in normal quail embryo fibroblasts in primary culture whereas it shows a high transcriptional activity after transformation by the Rous sarcoma virus. The 963 bp-long 5' flanking region is sufficient to drive the transcription of the chloramphenicol acetyltransferase reporter gene in a thermodependent manner when a thermosensitive version of pp60v-src is used. Deletion and point mutation analyses of the promoter show that the v-src response requires at least two separate elements: PUR and AP-1, located respectively at positions -167 to -152 and -25 to -19 relative to the single transcription initiation site. In addition, the binding of specific nuclear factors to these responsive elements correlates with the T64 promoter activation.

The long repetitive polypurine/polypyrimidine sequence (TTCCC)48 forms DNA triplex with PU-PU-PY base triplets in vivo

Polypurine/polypyrimidine repetitive sequences occur with high frequency in eucaryotic genomes, particularly around transcription units. Since such sequences are known to adopt triple stranded-structures under appropriate conditions in vitro, it is of major interest to know if they occur in vivo, and thus if they can have some biological importance by inducing structural constraints in the genomic DNA. To this end, we have isolated a (TTCCC)48 sequence, present in the promoter of an avian gene, and tested its ability to form PU-PY-PY and PU-PU-PY triple helices in vitro, through the oligonucleotide gel shift technique and single strand-specific nuclease footprinting. We have then developed an oligonucleotide protection assay, which can be adapted to in vivo investigations. This strategy leads us to conclude that in vivo conditions allow preponderant formation of triplex of the PU-PU-PY class.