Skip to main content
. 2009 Mar;73(1):36–61. doi: 10.1128/MMBR.00026-08

FIG. 1.

FIG. 1.

The T-box RNA regulatory system. (A) Structural model of the B. subtilis tyrS T-box leader RNA. The T-box element present in the B. subtilis tyrS leader region was originally described by Grundy and Henkin (see reference 43). The standard T-box leader RNA arrangement consists of three major elements, stem I, stem II, and stem III plus the stem IIA/stem IIB pseudoknot, and the competing terminator and antiterminator structures. The specifier loop, an internal bulge in stem I, contains the specifier sequence (boxed UAC residues complementary to the anticodon sequence of tRNATyr); the conserved purine (an adenine) following the specifier sequence is inside a green circle. The T-box sequence is unpaired in the terminator form and is paired in the antiterminator form (the antiterminator is shown to the right of the terminator). The sequence highlighted in blue shows the nucleotides involved in the antiterminator structure. The antiterminator structure has a bulge that interacts with the unpaired residues at the acceptor end of an uncharged tRNA. Nucleotide conservation in all 722 T-box sequences analyzed was evaluated using a multiple sequence alignment obtained from the Rfam database (42), and residues are color coded accordingly. (B) Model of the regulatory alternatives for the T-box mechanism. During the transcription of a leader region by RNA polymerase (red ovals), the nascent RNA folds into a structure competent for binding of the cognate tRNA at two sites. The binding of uncharged tRNA (top) to both the specifier sequence and the antiterminator bulge stabilizes the antiterminator (green RNA segment), preventing the formation of the terminator. This allows transcription to proceed into the downstream-regulated coding sequence (blue box). Charged tRNA (represented by Tyr attached to the 3′ end of the tRNA) can interact with the specifier sequence but cannot interact with the antiterminator; a failure to stabilize the antiterminator allows the formation of the terminator helix (red RNA segment), and transcription is terminated before the downstream coding region can be transcribed. Conserved elements of T-box RNAs are stem I (black), stem II (orange), the stem IIA/stem IIB pseudoknot (light blue), and stem III (purple).