An instance of this class defines a transcription unit, meaning a set of one or more genes that are transcribed to produce a single messenger RNA. The differences between a transcription unit and an operon are (1) Technically, an operon must include more than one gene. (2) An operon may include more than one promoter. For operons that include multiple promoters, a transcription unit is defined for each promoter.
A transcription unit should have as its components one or more genes, zero or one promoter, and zero or more terminators. Transcription factor binding sites need not be components of a transcription unit. A one-to-one correspondence is defined between a transcription unit and a promoter plus set of genes. Promoters can be omitted if the transcription start site is unknown, in which case the correspondence is between a transcription unit and a set of genes. Multiple terminators can be included in a transcription-unit, but typically a separate transcription unit is defined for each different set of transcribed genes (an exception is made for attenuators -- generally no transcription unit is defined for the truncated transcript produced by attenuation, even if it includes the gene for a leader peptide).
a DNA segment
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