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 th e genome of mammals, in which unr, like N-ras, is ubiquitously expressed. T o determine at what point in evolution the unr/N-ras linkage was created, a nalysis of nucleic acids by Southern and Northern blotting was performed, a llowing 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 antibodi es, a potential relation between unr protein levels and cellular processes in which N-ras is implicated. A positive correlation in the proliferation o f 3T3 cells, but not differentiation of PC12 cells induced by nerve growth factor (NGF), was found. To study the nucleic acid-binding properties of un r, a protein with multiple repeats of a nucleic acid-binding motif, we expr essed the long splicing isoform in a eukaryotic cell line and purified it i n native form. The results obtained-a high affinity of unr for single-stran ded DNA and RNA and lower affinity for double-stranded DNA without regard t o nucleic acid sequence, and its intracellular localization in both the nuc lear and non-nuclear compartments, together with its ubiquious expression i n mammalian tissues-provide molecular information about the function of one of the closest gene tandems in mammalian cells (unr-N-ras).