Splicing of SV40 late mRNA is a post-transcriptional process

Abstract

THE synthesis and metabolism of SV40 mRNA is being used as a model system to study RNA transcription and processing in animal cells. We and others have recently reported the finding of a novel mechanism of viral RNA maturation in SV40-infected cells^1–7 in which the leader sequences (those found at the 5′ ends of the main bodies of SV40 late mRNAs) and the adjacent coding sequences are transcribed from non-contiguous segments of the viral DNA. Similar observations have been reported for adenovirus mRNAs^8,9. This process is known as splicing and may proceed by several possible routes. These include: intermolecular ligation of RNA; deletion in the DNA template of the intervening sequences; looping out of intervening DNA sequences so that the RNA polymerase could skip over short distances; and deletion of the appropriate intervening RNA sequences at the post-transcriptional level. We present here an analysis of R-loop structures¹⁰ formed between nuclear and cytoplasmic poly(A)⁺ RNAs and linear SV40 DNA, which provides support for the last of these possibilities⁴⁰. A similar analysis has been carried out for β-globin mRNA¹⁷.