Once the hairpin secondary structure in the RNA immediately preceding the trp attenuation site was associated with transcription termination, several critical experiments were apparent. One was to examine the consequences of changing the sequence of the region. Mutations changing the stability of the hairpin might be expected to alter termination efficiency. A second and easier experiment was to examine naturally occurring sequence variants. Because related bacterial strains must have evolved from a common predecessor, they probably share the same basic mechanism of trp regulation. If the leader sequences of their trp operons possess structural features in common, these features are likely to be important in the regulatory mechanisms. Indeed, this was found. The trp operon in Salmonella typhimurium and Serratia marcescens also contain leader regions. Both code for a leader peptide containing several tryptophans, and both mRNAs are capable of forming a hairpin just before a string of U’s located about 20 bases ahead of the trpE gene.

Close examination of the leader region of the trp operon of E. coli, the trp operons from other strains of bacteria, and other amino acid biosynthetic operons has revealed two additional facts. First, the pep tides encoded by the leader regions always contain one to seven of the amino acid residues synthesized by that operon. Second, the leader mRNAs possess at least four regions that can form intramolecular base-paired hairpins (Fig. 1). Here they are numbered 1, 2, 3, and 4. The pairing of 3 and 4 forms the hairpin discussed above that is required for termination of transcription. Additionally, region 2 can base pair either with region 3 or with region 1. Likewise, region 3 can base pair with region 2 or 4. The particular hybrids that form on a leader molecule determine whether transcription of that molecule will terminate as RNA polymerase passes the attenuation region. As we will see, the pairing choice is regulated by the position on the leader of a ribosome translating the leader peptide.

Fig1. The two possible structures that the second half of the trp leader region is capable of assuming. Above is indicated the leader region with the complementary regions. Opposed arrows at the same height above the leader are homologous and can base pair. The wavy line below the leader mRNA is the region encoding the leader peptide.

اخر الاخبار

اخبار العتبة العباسية المقدسة

جولة تفقدية لمتابعة خطة تنظيم مداخل المعزّين المشاركين في عزاء ركضة طويريج

لتعزيز التعاون الأكاديمي والبحثي.. جامعة الكفيل توقع اتفاقيتي توأمة مع جامعتين عراقيتين

المجمَع العلميّ يواصل إقامة مجلسه العزائي بذكرى شهادة الإمام الحسين (عليه السلام) في النجف الأشرف

قسم حفظ النظام يواصل تنفيذ خطته لتنظيم سير المواكب والزائرين في شهر المحرم

المزيد

الآخبار الصحية

لهذا السبب.. إغلاق 25 مركزًا لعلاج الإدمان في مصر ؟

خلافا للاعتقاد الشائع.. علماء يكشفون أسرار بذور البطيخ ؟!

هذا ما يحدث لجسمك عند تناولك الطماطم بانتظام !

تعرف على أطعمة تسرّع "تعافي الجسم" من حروق الشمس

المزيد

الآخبار التكنلوجية

الثلوج تغطي الصحراء الأكثر جفافا في العالم!

بتلسكوب "ألما".. التقاط صورا غير مسبوقة لبدايات الكون !

هل هاتفك مراقب؟.. 5 علامات خطيرة تكشف التجسس عليك

دبي تطلق أول رحلة تجريبية للتاكسي الجوي

المزيد

مواضيع ذات صلة

Other Attenuated Systems: Operons, Bacillus subtilis and HIV

Coupling Translation to Termination

Early Explorations of the Hypersynthesis

The Serendipitous Discovery of trp Enzyme Hypersynthesis

Rapid Induction Capabilities of the trp Operon

The Aromatic Amino Acid Synthetic Pathway and its Regulation

How AraC Protein Loops and Unloops

DNA Looping and Repression of araBAD

Binding Sites of the ara Regulatory Proteins

Endoplasmic Reticulum-Associated Degradation and the Unfolded Protein Response

Protein Folding and Modifications in the Endoplasmic Reticulum

Protein Modifications