Mouse DNA sequences complementary to small nuclear RNA U1

A mouse genomic library was screened for sequences complementary to U1 nuclear RNA. Out of the eight clones tested, none contained more than one copy of U1. Six of them were identical and one of those (clone 0U1-XIII) was further analyzed. This latter clone contained no other gene for discrete species of small size RNA in the 8 Kb EcoRI fragment encoding U1. A 248 bp Bg1II fragment from 0U1-XIII encompassing the full length of U1 as well as flanking regions on both sides has been subcloned and sequenced in M13 phage. Although the coding region was 96.5% homologous to rat U1a RNA, there is no direct evidence that this clone is a true gene. 3' and 5' flanking sequences of this as well as other published clones have been searched for homologies and the results of this search are discussed.