Intergenic mRNAs. Minor gene products or tools of diversity?

The general model of gene expression implies that the units of genetic information, the genes, constitute the basis of the mRNA and protein products detected in living organisms. It is well known that in eukaryotic cells a single gene may give rise to various gene products due to alternative splicing and/or different positions of transcriptional start and termination. However, recent experimental results suggest that in addition to this variation in gene expression, another level of complexity may also exist as evidence for the presence of mRNAs that combine sequence information (exons) from distinct genes is accumulating. Moreover, the mechanisms that allow this production of intergenic mRNAs are starting to unravel and it appears that these follow two general pathways: (a) bypass of transcriptional termination, resulting in the generation of bicistronic pre-mRNAs; and (b) authentic trans-splicing events between pre-mRNAs of distinct genes.