The Ordered Transcription of RNA Domains Is Not Essential for Ribosome Biogenesis in Escherichia coli

Ribosome biogenesis is coupled to the transcription of ribosomal (r)RNAs, which enables an ordered and hierarchical assembly of the ribosome starting with the 5'-terminal domain. We constructed four circular permutants (CPs) of Escherichia coli rRNAs in which the original termini of 16S or 23S rRNAs were genetically connected, and new termini were created elsewhere within the same rRNAs (in helix 33 of 16S rRNA, or in helices 45, 63, and 78 of 23S rRNA). Unexpectedly, all CPs tested were able to rescue E coli strain Delta 7 prrn, which lacks all chromosomal rRNA operons. This result demonstrates that hierarchical assembly from the 5'-terminal domain of both 16S and 23S rRNAs is not essential for ribosomal formation in the cell. However, severe growth defects of all CPs were found in the absence of the DEAD box RNA helicase deaD, indicating that DeaD assists in the efficient assembly of each subunit in the cell.