Isolation of novel, transcriptionally active AP-1 binding sites: implications for cellular transformation

Increased AP-1 DNA-binding activity, in the context of TRE-binding, is not a consequence of Fos transformation. In this report we investigate the possibility of a change in binding site preference by vFosAP-1 compared with AP-1 from an untransformed cell. Fos binding sites were immunoselected from random sequence oligonucleotides using a pan Fos anti-serum with nuclear protein from quiescent FBRp75v-fos-transformed (FBR) and normal (208F) rat fibroblasts. The selected oligonucleotides were aligned by computer and a consensus described for the sequences bound by AP-1 from the two cell lines. The vFos binding site is shown to be a consensus TRE, whereas the sequence ACCACATC is described as the cellular Fos protein family consensus. We demonstrate that sequences differing from the TRE consensus can bind AP-1 and direct transcription. AP-1 DNA-binding activity differs between normal and transformed cells with several of the selected oligonucleotides. These sequences also demonstrate differential transcriptional activation between normal and transformed cells. In particular, the 208F consensus has no transcriptional activity in FBR cells. Further, EGF differentially influences the transcriptional activity of the oligonucleotides in 208F and FBR cells. Our results suggest that AP-1 may change its preferred binding site depending on the proteins available at any given time, the sequences flanking a non-consensus TRE or even the environment in which the cell exists. These differences in binding site preference and transcriptional activation may result in the increased transforming ability of the v-fos oncogene compared with the c-fos proto-oncogene and may extend the potential target genes beyond those with an AP-1 consensus binding site.

Fos-transformation activates genes associated with invasion

Fos oncoproteins transform cells by functioning as transcription factors. Over-expression of c-fos results in minimal morphological transformation while the two viral isolates, FBJ and FBR v-fos, result in full morphological transformation. Fos-transformed cells are serum dependent for proliferation but not for morphological transformation. To identify Fos target genes which might be involved in morphological transformation we screened a cDNA library constructed from RNA isolated from serum starved FBR-transformed cells with cDNA probes prepared from both FBR-transformed cells and untransformed parental fibroblasts, 208F. We identified 10 genes which are differentially expressed between FBR and 208F cells. One is a novel gene. Nine are upregulated in c-fos- and FBJ-transformed cells and also in mutant c-Ha-Ras-transformed 208Fs. All nine of the upregulated genes have been associated previously with invasion or metastasis. We demonstrate that the FBR-transformed cells are invasive in an in vitro assay and that their ability to invade is enhanced by platelet derived growth factor. We conclude that the fos oncogenes target genes involved in morphological transformation, and invasion.

Transformation by the fos or jun oncogene does not increase AP-1 DNA-binding activity

The c-fos and c-jun proto-oncogenes encode components of the transcription factor AP-1. To determine whether transformation by the v-fos or v-jun oncogene results in alterations in the level or regulation of this factor, we have characterized AP-1 DNA-binding activity in nuclear extracts prepared from v-fos- and c-fos-transformed rat fibroblast cell lines and v-jun-transformed chicken embryo fibroblasts under various growth conditions. During proliferation, the level of AP-1 DNA-binding activity does not differ among the v-fos, c-fos, or v-jun-transformed cells and their normal progenitors, despite constitutive overexpression of the corresponding oncoproteins. Therefore, although necessary, it is not likely that an increase in DNA binding is sufficient for fos or jun transformation. Normal rat and chicken fibroblasts demonstrate very low levels of AP-1 DNA-binding activity when quiescent, and upon serum stimulation a biphasic increase is observed. A similar cyclical pattern is seen in v-fos-transformed cells, but in v-jun-transformed cells AP-1 DNA-binding activity does not fluctuate in response to serum stimulation, which suggests that this level of control may be exerted through the Jun component of the AP-1 complex.

The Aspergillus niger niaD gene encoding nitrate reductase: upstream nucleotide and amino acid sequence comparisons

The Aspergillus niger niaD gene has been sequenced and the inferred nitrate reductase (NR) protein found to consist of 867 amino acid residues (97 kDa). The gene is interrupted by six small introns, as deduced by comparison with the niaD gene of Aspergillus nidulans. The positions of these putative introns are conserved between the two fungi, although the sequences are dissimilar. The niiA gene, encoding nitrite reductase, the second reductive step in the nitrate assimilation pathway, is tightly linked to niaD and divergently transcribed in A. niger, similar to the general organisation in the related fungi, Aspergillus oryzae and A. nidulans. The nucleotide (nt) sequences of the intergenic region between niiA and niaD (excluding the ATG translation start codon) of A. niger (1668 nt) and A. oryzae (1575 nt) were determined and compared with the previously determined A. nidulans (1262 nt) sequence. No striking extended nt regions of homology are observed in spite of the fact that transgenic strains with fungal niaD or the two control genes required for induction and repression show virtually normal regulation. Fungal NR shows considerable aa homology with higher plant NR, particularly within the co-factor domains for flavin adenoside dinucleotide, heme and molybdopterin cofactor.

Nitrate reductase and nitrite reductase transcript levels in various mutants of Aspergillus nidulans: confirmation of autogenous regulation

The regulation of the expression of the A. nidulans niiA and niaD genes (encoding nitrite reductase and nitrate reductase activities, respectively) was investigated by Northern blotting. It was demonstrated that expression of the niiA and niaD genes is controlled at the level of mRNA accumulation and that mutations within the nirA and areA regulatory genes, as well as certain mutations within niaD itself or cnxE (for its molybdenum cofactor), markedly affect niiA and niaD transcript levels.

Heterologous expression and regulation of the Neurospora crassa nit-4 pathway-specific regulatory gene for nitrate assimilation in Aspergillus nidulans

The nirA gene of Aspergillus nidulans and the nit-4 gene of Neurospora crassa appear to be equivalent pathway-specific regulatory genes which mediate nitrate induction of nitrate reductase and nitrite reductase (NR and NiR) activities. We have transformed the nit-4 wild-type (wt) gene into the A. nidulans loss-of-function (pleiotropic negative) nirA 1 mutant strain. The nit-4 gene was found to complement the nirA 1 mutation, thus permitting the nirA 1 mutant strain to grow on nitrate or nitrite as the sole source of nitrogen. Integration of the nit-4 gene in transformants appears to have occurred at a number of 'ectopic', i.e. non-nirA, sites. Nitrate is required for the induction of NR activity in nit-4-transformed strains whilst NR production remains markedly subject to nitrogen-metabolite repression. However, NR levels are modestly higher than wt under all growth conditions.