Leader region

Richardson JH, Child LA, Lever AM (1993) Packaging of human immunodeficiency virus type 1 RNA requires cis-acting sequences outside the 5 leader region, J Virol 67 3997 005 Roberts MR, Qin L, Zhang D, Smith DH, Tran AC, Dull TJ, Groopman JE, Capon DJ, Bym RA, Finer MH (1994) Targeting of human immunodeficiency virus-infected cells by CD8-I- T lymphocytes armed with universal T-cell receptors. Blood 84 2878-2889 Rooney CM, Smith CA, Ng CY, Loftin S, Li C, Krance RA, Brenner MK, Heslop HE (1995) Use of gene-modified virus-specific T lymphocytes to control Epstein-Barr-virus-related lympho-proliferation. Lancet 345 9-13... 

The trp operon attenuation mechanism uses signals encoded in four sequences within a 162 nucleotide leader region at the 5 end of the mRNA, preceding the initiation codon of the first gene (Fig. 28-21a). Within the leader lies a region known as the attenuator, made up of sequences 3 and 4. These sequences base-pair to... 

Regulatory sequence 1 is crucial for a tryptophan-sensitive mechanism that determines whether sequence 3 pairs with sequence 2 (allowing transcription to continue) or with sequence 4 (attenuating transcription). Formation of the attenuator stem-and-loop structure depends on events that occur during translation of regulatory sequence 1, which encodes a leader peptide (so called because it is encoded by the leader region of the mRNA) of 14 amino acids, two of which are Trp residues. The leader peptide has no other known cellular function its synthesis is simply an operon regulatory device. 

Transcription Attenuation How would transcription of the E. coli trp operon be affected by the following manipulations of the leader region of the trp mRNA ... 

Schematic diagram of the repressor control of trp operon expression. The trp promoter (P) and trp operator (O) regions overlap. The trp aporepressor is encoded by a distantly located trpR gene. L-Tryptophan binding converts the aporepressor to the repressor that binds at the operator locus. This complex prevents the formation of the polymerase-promoter complex and transcription of the operon that begins in the leader region (trpL). Only a fraction of the transcripts extends beyond the attenuator locus in the leader region. The regulation of this fraction is discussed in the text.

The leader region for the tryptophan operon. The region of the leader RNA containing the hypothesized leader polypeptide is shown. The translation start of the trpE protein is also shown. 

Leader region. The region of an mRNA between the 5 end and the initiation codon for translation of the first polypeptide chain. 

Toward the 5 end of mRNA, there is a region of 20 or so nucleotides before the initiation codon AUG is reached. This leader region contains a sequence responsible for interaction of the mRNA with the 30S subunit. It is known as the Shine-Dalgarno (S-D) sequence, and it can bind to a complementary sequence at the 3 end of the 16S rRNA to position the 30S subunit appropriately for initiation. Other sequences in the leader region are possibly involved in the overall process of initiation of translation, which involves also the binding of the appropriately charged methionyi-tRNA opposite the AUG codon. 

Question During translation, as the ribosome moves along the mRNA, it leaves the leader region (containing the ribosome binding site) empty. Is it possible for initiation of a new polypeptide chain to occur before completion of a previous one ... 

In initiation of translation, the mcthionyl-tRNA species tRNAf 101 is used exclusively. Presumably, the unique nucleotide sequence of this species is required for the initial interaction with the small ribosomal subunit, the leader region of mRNA, initiation factors, and GTP to give the first initiation complex. It is unlikely that formylation (which can occur only with mcthionyl-tRNA el) is essential for initiation, as this is not the case in eukaryotes. It thus appears that tRNAjJJ - 1 is excluded from initiation because essential structural requirements in the RNA are not met. 

Gong E, Yanofsky C. A transcriptional pause synchronizes translation with transcription in the tryptophanase operon leader region. J. Bacteriol. 2003 185 6472-6476. 

McGraw A, Bevilacqua PC, Babitzke P. TRAP-5 stem-loop interaction increases the efficiency of transcription termination in the Bacillus subtilis trpEDCFBA operon leader region. RNA. In press. 

Figure 31.34. Leader Region of JKPmRNA. (A) The nucleotide sequence of the 5 end of t p mRNA includes a short open reading frame that encodes a peptide comprising 14 amino acids the leader encodes two tryptophan residues and has an untranslated attenuator region (blue and red nucleotides). (B and C) The attenuator region can adopt two distinct stem-loop structures.

Gene expression can also be regulated at the level of translation. In prokaryotes, many operons important in amino acid biosynthesis are regulated by attenuation, a process that depends on the formation of alternative structures in mRNA, one of which favors transcriptional termination. Attenuation is mediated by the translation of a leader region of mRNA. A ribosome stalled by the absence of an aminoacyl-tRNA needed to translate the leader mRNA alters the structure of mRNA so that RNA polymerase transcribes the operon beyond the attenuator site. 

How does the level of tryptophan alter transcription of the trp operon An important clue was the finding that the 14-amino-acid leader peptide includes two adjacent tryptophan residues. A ribosome is able to translate the leader region of the mRNA product only in the presence of adequate concentrations of tryptophan. When enough tryptophan is present, a stem-loop structure... 

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