Many genes have multiple alternative transcripts with a different TSS.
When gene structure becomes better annotated, we might discover that many genes have multiple alternative 5' or 3' UTRs for transcription, so that two neighboring genes may become overlapping with each other in certain transcribed forms.
All bacteria contain a primary σ factor that directs transcription from the major class of bacterial promoters; in addition, most bacteria contain multiple alternative σ factors that direct transcription from specialized promoters in response to stress or alterations in growth state (Gruber and Gross, 2003; Osterberg et al., 2011; Guo and Gross, 2014).
The majority of mammalian genes generate multiple transcript variants and protein isoforms through alternative transcription and/or alternative splicing, and the dynamic changes at the transcript/isoform level between non-oncogenic and cancer cells remain largely unexplored.
The NHS gene appears to have multiple isoforms as a result of alternative transcription, but a cellular function for the NHS protein has yet to be defined.
The majority of mammalian genes produce multiple transcripts resulting from alternative splicing (AS) and/or alternative transcription initiation (ATI) and alternative transcription termination (ATT).
The remaining ∼33 % is of complex events that include mutually exclusively alternative transcription start sites and multiple polyadenylation sites [8], [9].
However, there are also complex AS types combined with at least two simple forms or some other low frequent forms such as mutually exclusive exons, alternative transcription start sites, and multiple polyadenylation sites in eukaryotes [ 7].
Indeed, there are estimated to be well over 100 mRNA products, as well as multiple proteins transcribed from these genes, due to alternative transcription start sites and splicing of precursor molecules (Francis et al., 2011).
Alternative transcription initiation and splicing of mRNAs can produce multiple transcripts from a single gene; alternative translation initiation sites can then produce different proteins from each of these transcripts, and these protein variants can be targeted to different compartments in the cell.
