A transposon could carry DNA with phage-like functions rather than a simple antibiotic resistance gene. Such a transposon would be able to travel freely from cell to cell autonomously as a virus. Phage Mu first drew attention by its ability to generate mutations in infected cell populations. These mutations were generated by its insertion into various bacterial genes. Careful mapping of many Mu phage insertions into the β-galactosidase gene showed that Mu inserted without high specificity in its target sequence. Thus, Mu behaved like a lambda phage with little sequence specificity in its choice of the bacterial att site.

Two facts, however, indicate that Mu is more like a transposon than like phage lambda. First, Mu duplicates five bases of chromosomal DNA upon its insertion. Most transposons generate such duplications. These arise as a result of staggered nicking of the target sequence followed by insertion of the transposon and replication (Fig. 1). Second, Mu does not excise from the chromosome and replicate in the cytoplasm. Even though an induced cell may yield a hundred Mu phage upon lysis, never during the lytic cycle are any free Mu DNA molecules observed. The Mu DNA is replicated only by transposition. Packaging of the Mu takes place directly on the integrated DNA. A dramatic demonstration of replication by transposition is provided by Southern transfers of DNA taken from a Mu lysogen before and after induction of the phage. Before induction, only a single restriction fragment of the host DNA contains sequences homologous to a restriction fragment including the end of Mu, but half an hour after induction of Mu growth, many restriction fragments contain these sequences (Fig. 2).

Fig1. Insertion of a transposon into a site with staggered nicks followed by filling in of the gaps generates a duplication of the sequence included between the nicks.

Fig2. DNA taken from a Mu lysogen before and after phage induction used in a Southern transfer.

Mu packages its DNA right out of the chromosomal insertions. Part of the head structure recognizes a sequence near the left end of the DNA, reaches out beyond the phage 50 to 150 base pairs, and begins packaging. A headfull of DNA is packaged, and the remaining DNA is then cleaved and the tail is attached. Since a Mu phage headfull of DNA is slightly larger than the genome of the phage, the packaged DNA usually extends beyond the right end of the phage and includes about 3,000 base pairs of bacterial DNA as well. The headfull hypothesis explains the fact that a Mu phage carrying an insertion packages a smaller amount of bacterial DNA roughly equal in amount to the size of the insertion.

The sequence heterogeneity that results from the headfull mode of packaging is dramatically displayed when heteroduplexes of phage genomes are analyzed in the electron microscope. Since it is highly unlikely that the two strands from the same phage will rehybridize upon formation of the heteroduplex, two unrelated strands will associate. The phage DNA portions of the strands will, of course, be complementary and will form a duplex. The strands of bacterial DNA on the right end, however, are unlikely to be complementary, and these will remain single-stranded and are observed as “split ends.” The similar split ends from the left end are too small to be clearly observed.

اخر الاخبار

اخبار العتبة العباسية المقدسة

بيان لمكتب المرجعية العليا حول نشر صور السيد السيستاني في الأماكن العامة

متحف الكفيل: إنجاز المرحلة الرابعة من مشروع متحف العتبة العسكرية المقدسة

تضمن عرضًا مسرحيًّا.. شعبةُ الخطابة النسويّة تقيم مجلس عزاء بذكرى شهادة الإمام الحسن (عليه السّلام)

الأمين العام للعتبة العبّاسية المقدّسة يتفقد مجموعة مواكب أمّ البنين (عليها السلام) لخدمة الزائرين

المزيد

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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

المزيد

الآخبار التكنلوجية

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

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بشرى لمرضى الشلل.. شريحة في الدماغ قد تتيح التحكم بالأدوات

مدينة غارقة.. هل يخفي قاع البحر الكاريبي سرّ حضارة مفقودة؟

المزيد

مواضيع ذات صلة

Membrane Proteins

In vitro Transposition, Threading or Global Topology?

An Invertible Segment of Mu Phage

Inverting DNA Segments by Recombination, Flagellin Synthesis

Discovery of Tn Elements

Structure and Properties of IS 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