While IS elements were under investigation, another category of genetic elements was also being studied. These were plasmids that carry genes encoding proteins that confer resistance to antibiotics. They are called R-factors.

Not long after antibiotics began to be widely used, many bacterial isolates from human infections were found to be resistant to one or more of the drugs. These isolates were not resistant by virtue of a mutation altering the cellular target of the antibiotics; instead, they synthesized specific proteins or enzymes, which conferred resistance to the cells either by detoxifying the antibiotics or by blocking their entry into the cell. For example, penicillin-resistant strains were found to synthesize a β-lactamase that opens the lactam ring of penicillin and renders it harmless to the cells (Fig. 1).

Fig1. The structure of the lactam ring of penicillins and related antibiotics and their structure following ring opening by β-lactamase. Different penicillins possess different R groups.

Although many R-factors were fertile and could transfer themselves to other cells by conjugation, the speed with which different drug resistances appeared on R-factors was astonishingly rapid. Soon after the introduction of a new antibiotic, R-factors that carried genes conferring resistance to the new antibiotic would be found in many geographic locations. Unfortunately this has created great problems in the treatment of infections. The drug-resistance factors rapidly pick up genes encoding resistance to the most commonly used drugs, and soon treatment of infections becomes difficult. Now physicians use restraint in prescribing antibiotics to slow the spread of drug-resistance genes.

The ability of drug-resistance genes to spread rapidly suggested that they could hop from one R-factor to another or to other DNA sequences such as phage or the bacterial chromosome. One demonstration of this property came from an attempt to construct a lambda phage carrying the kanamycin-resistance gene from an R-factor.

The objective of the experiment was to generate a λkan transducing phage by forcing lambda to insert at random locations, and then excise. A small fraction of the excised phage would have picked up the kanamycin genes and be able to transduce cells to Kan^r. Cells deleted of attB and carrying a kanamycin-resistance R-factor were infected with lambda. This lambda phage was deleted of part of the b2 region so that the DNA of the resulting kanamycin-transducing phage would not be too large to be packaged. To permit the phage to integrate via the Int pathway, the deletions chosen did not extend into the att region. As seen in the previous chapter, a small fraction of the resulting lysogens should then contain a lambda inserted into DNA adjacent to the kanamycin-resistance gene(s) by virtue of integrating into sites that weakly resemble lambda’s normal integration site (Fig. 2). Upon induction, a small fraction of the phage were expected to excise incorrectly, pick up the adjacent kanamycin-resistance genes, and be capable of transducing kanamycin resistance to other cells.

Fig2. The structure of λ-kan^r expected if the transducing particles were generated by the same mechanism by which λ-gal and λ-bio transducing phages are produced, and the structure of λ-kan actually found.

Indeed, kanamycin-transducing phage were isolated. The phage possessed several unexpected properties, however. First, the additional DNA in these phage was not located immediately adjacent to attP. Second, kanamycin-resistant transformants obtained by infecting cells with the λkan phage were lysogenic for lambda only if they contained an attB. These properties can be understood as follows. The DNA coding for kanamycin resistance originally hopped or was copied onto lambda by a recombination event rather than arriving there after an improper phage excision of the type that produces gal and bio transducing particles. Therefore the foreign DNA could be located anywhere on the phage and need not be immediately adjacent to attP. Second, a copy of the sequence encoding the kanamycin resistance could also hop or be copied from the phage onto the chromosome.

The DNA responsible for conferring the kanamycin resistance was examined by DNA heteroduplexes. It was found to be 5,400 base pairs long and flanked by inverted repeated sequences 1,500 base pairs long. Some of the drug-resistance genes studied on the R-factors themselves

also possessed similar flanking sequences. Further study showed that the repeated sequences flanking the drug-resistance genes were transferred when the drug-resistance gene hopped. Finally, it was found that the repeated elements at the ends of many of the drug-resistance genes possessed DNA sequences essentially identical to known IS elements. Thus it appears that two IS elements can flank a gene and then facilitate copying the composite element into other DNA locations by the same transposition reactions as are used by IS elements themselves. Sequences with these general properties and structure are called transposons, or Tn elements (Table 1). The element described in this section is called Tn5, and the IS sequences associated with it are called IS50 so as not to be confused with IS5.

Table1. Properties of Some Selected Transposons

اخر الاخبار

اخبار العتبة العباسية المقدسة

استعدادًا لزيارة الأربعين العتبة العباسية المقدسة تعقد اجتماعًا لتنظيم عمل الهيئات التطوّعية في الصحن الشريف

قسم الشؤون الخدمية يواصل تنفيذ أعمال الوقاية ومكافحة الحشرات في مواقع العتبة العباسية المقدسة

المجمَع العلمي للقرآن الكريم يواصل تنفيذ مشروع بيوت النور القرآني

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

المزيد

الآخبار الصحية

تسارع الشيخوخة.. متى يبدأ وأي الأعضاء أكثر تأثرا؟

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"

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

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

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

المزيد

مواضيع ذات صلة

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

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