Most of the bacterial IS sequences contain two characteristic elements. First, a sequence of 10 to 40 base pairs at one end is repeated or nearly repeated in an inverse orientation at the other end. This is called IR, for inverted repeat. Second, the IS sequences contain one or two regions that appear to code for protein because they possess promoters, ribosome-binding sequences, ATG (AUG) codons, and sense codons followed by peptide chain termination codons. If proteins were made under control of these sequences, they would likely be involved with the process of copying the IS into a new position in the process of transposition. Indeed, one of the gene products is generally called the transposase. As only extremely low quantities of a transposase are needed, its transcription and translation are specially designed to be inefficient. Additionally, some premium is placed on possessing a mini mum size, and genes of a transposon frequently overlap. For example, in IS5 two oppositely oriented reading frames overlap. The shorter is contained entirely within the longer (Fig. 1).

Fig1. The organization of IS5. The large and small proteins are in the same codon register.

A high expression of the IS genes most likely would stimulate a high rate of transposition into new chromosomal sites. Since IS transposition rates appear low, something must protect IS genes from readthrough transcription and translation if the sequence has inserted into an active gene of the host cell. IS5 possesses six peptide chain termination codons flanking the protein genes—one at each end of the transposon, one in each reading frame. Additionally, the left end of the IS5 contains a transcription termination sequence oriented so as to prevent transcription from adjacent bacterial genes from entering the IS in the reading direction of the transposase.

Not only do some IS sequences copy themselves or jump into new DNA locations, but they also stimulate the creation of deletions located near the IS elements. These deletions have one end precisely located at the end of an IS element and apparently arise in abortive transposition attempts that begin with transposase nicking the DNA at one of the boundaries of the IS element (Fig. 2). Such a facile creation of deletions in any selected region is highly useful to geneticists, for, as we have seen, a set of such deletions can be used for fine structure mapping.

Fig2. One mechanism by which nicking DNA can stimulate generation of deletions.

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قسم الشؤون الفكرية يبحث التعاون في مجال البحث مع كلية التقنيات المتعددة في نيجيريا

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9 مسببات شائعة للالتهاب المزمن قد تهدد صحتك

باحثون يطورون علاجا لهشاشة العظام.. النتائج مذهلة

تحمي من أمراض عديدة.. 6 فوائد لتناول البطاطا الحلوة

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عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

المعلمون في خطر.. خاصية جديدة في "تشات جي بي تي" تشرح الدروس

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مواضيع ذات صلة

Discovery of Tn Elements

IS Elements in Bacteria

Holliday Structures and Branch Migration in Integration

Structure of the Intasome

In vitro Assay of Integration and Excision

Inhibition By a Downstream Element

Demonstrating Xis is Unstable

The Protein Kinase Domain

Posttranslational Modifications

The Double att Phage, att2

Transducing Phage Carrying Genes Other than gal and bio

Incorrect Excision and gal and bio Transducing Phage