Nucleic Acids Research

Northern blot analysis reveals that total RNA from human fibroblastoid cells (MG 63) induced with poly(I).poly(C) under conditions of IFN-,B+ production, contains predominantly a ±1,200 nucleotide long poly (A) mRNA (mRNA.M) which hybridizes with a Hu IFN-J3 cDNA specific probe. But hybridization with this probe also enabled the detection of a polyadenylated RNA (RNA.I) with a length of between 3.5 kb-3.8 kb, representing 0.6% of the total hybridizable cellular RNA in superinduced cells. Mapping shows that the RNA.I contains all the sequence information present in mRNA.M. Furthermore, it also hybridizes to sequences, located downstream from the IFN-)3 gene up to 2.5 kb beyond its poly A attachment site, while no hybridization to fragments located upstream of the IFN-,$ mRNA cap site was observable. Hence this RNA.I corresponds to a transcript that starts at the same position as the major mRNA.M but which extends up to 2.5 kb beyond the 3'-end of mRNA.M where another polyadenylation signal is located. INTRODUCTION Expression of the hujman IFN-9 gene in fibroblastic cells is dependent upon a complex control system. The possibility of quantifying the end product using very sensitive biological assays based on the high specific activity of IFN has allowed detailed studies (see reviews in 1 and 2). Several phenomena such as priming, poly(I).poly(C) induction, shut-off, refractory period, and superinduction are known to govern expression and are mainly (or exclusively) operative at the transcriptional level (3-5). This system of IFN-S gene expression can now be studied at the molecular level because of the recent advances made in the cloning and sequencing of the corresponding cDNA (6-9). The gene is present as a single copy per haploid genome (10-12) and is located on chromosome 9 (13). The sequence of a 285 nucleotide long 5'-flanking zone and of 714 nucleotides corresponding to the 3'-flanking zone have been determined (14-16) confirming that the IFN-.B contains no intervening sequences [as is also true for Hu IFN-oc genes, some histone genes, heat shock protein genes in Drosophila and protein IX from adenovirus, reviewed © I RL Press Limited, Oxford, England. Volume 1 1 Number 9 1983