An isolated pseudogene related to the 5S RNA genes in Neurospora crassa

Abstract

DNA segments which do not code for functional products but are similar in sequence to DNA of known function are called ‘pseudogenes’. Most pseudogenes discovered so far are related to members of gene families: 5S RNA (5S) genes in Xenopus^1–3, globin genes in various mammals^4–6, actin genes in Dictyostelium⁷ and small nuclear RNA genes in man⁸. Presumably, in the absence of strong selective forces, redundant genes drift apart and occasionally become nonfunctional. Of course, mechanisms which maintain homogeneity in multigene families should tend to prevent the occurrence of pseudogenes. There is ample evidence for ‘horizontal’ or ‘concerted’ evolution in tandemly repeated gene families^9–11. The 5S pseudogenes of Xenopus laevis, of which there is a copy in every tandemly repeated unit, may be a rare example of a defective gene not eliminated from the family by unequal crossing-over or gene conversion events. We have recently found evidence for limited concerted evolution in a dispersed gene family, the 5S genes of Neurospora crassa¹²: both identical and highly divergent 5S coding regions were discovered. We report here that we have found a pseudogene related to the Neurospora 5S genes. This pseudogene, N5SP1, contains a segment of DNA almost identical to the first 50 nucleotides of the major species of Neurospora 5S RNA. The sequences flanking this region are totally different from sequences found within or adjacent to full-length 5S genes. Hybridization experiments suggest that N5SP1 is not transcribed. The existence of this pseudogene is consistent with our transposition model for horizontal evolution in dispersed gene families¹².