The unr gene: evolutionary considerations and nucleic acid-binding properties of its long isoform product.

The unr transcription unit is located just upstream of the N-ras gene in the genome of mammals, in which unr, like N-ras, is ubiquitously expressed. To determine at what point in evolution the unr/N-ras linkage was created, analysis of nucleic acids by Southern and Northern blotting was performed, allowing us to track the presence of the unr gene to the start of vertebrate evolution and the unr/N-ras linkage to the time at which the reptilian and bird lines diverged. We have investigated, with specific anti-unr antibodies, a potential relation between unr protein levels and cellular processes in which N-ras is implicated. A positive correlation in the proliferation of 3T3 cells, but not differentiation of PC12 cells induced by nerve growth factor (NGF), was found. To study the nucleic acid-binding properties of unr, a protein with multiple repeats of a nucleic acid-binding motif, we expressed the long splicing isoform in a eukaryotic cell line and purified it in native form. The results obtained-a high affinity of unr for single-stranded DNA and RNA and lower affinity for double-stranded DNA without regard to nucleic acid sequence, and its intracellular localization in both the nuclear and non-nuclear compartments, together with its ubiquious expression in mammalian tissues-provide molecular information about the function of one of the closest gene tandems in mammalian cells (unr-N-ras).