In many cases, newly transcribed RNA is not ready for duty; instead, it is a precursor of mature RNA and must undergo posttranscriptional RNA processing to become functional. Also known as RNA maturation, posttranscriptional processing falls into seven categories:
cleaving and trimming
3′ nucleotide addition
capping (modifying the 5′ end of pre-mRNA)
polyadenylation (adding a sequence of adenosines to the 3′ end of pre-mRNA)
editing (altering the coding sequence of pre-mRNA)
The exact nature of every maturation process depends on the life form and the type of RNA (rRNA, tRNA, mRNA). This chapter describes the seven types of processing in turn, noting differences among life forms. At chapter's end, some processing pathways are summarized.
Cleaving and Trimming
Some fresh transcripts contain extraneous sequences, and some contain two or more sequences that need to be separated. Endonucleases may cleave transcripts into smaller pieces to remove end sequences or internal spacers. In some instances, exonucleases trim the ends, removing nucleotides one by one.
Large rRNAs are encoded by genes whose transcripts are cleaved in multiple steps to yield smaller RNAs. In bacteria, each of the six copies of the rRNA gene contains 16S, 23S, and 5S rRNA sequences and one or two tRNAs; the functional and nonfunctional sequences are interspersed (Figure 8.1). Processing by ribonucleases (RNases) begins before transcription is complete.