Mcrobial genetics

1. ara operon
presentation
Dr. P. Suganya
Assistant Professor
Department of Biotechnology
Sri Kaliswari college (autonomous), sivakasi

2. operon
FRANCIS JACOB And JACQUES MONAD 1961
Explained that gene regulation is by operon model
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3. “
The Operon Consist of regulatory gene structural
gene operator and promoter site This 3 basic DNA
components
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4. 3 BASIC DNA COMPONENTS
▸ Structural genes – The genes that ara co-regulated
by the operon
▸ Operator –A segment of DNA to which a repressor
binds
 Promoter – A Nucleotide sequence that enables a
gene to be transcribed
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5. Ara Operon
The L-arabinose operon , also called ara operon ,is a
gene sequence encoding enzymes needed for tha
catabolism of arabinose to xylulose 5- phosphate ,an
intermediate of the pentose phosphate pathway. It
has both positive and negative regulation . The
genes araBAD and araC,are transcribed in opposite
directions
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6. Structure of ara operon
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7. “
arabinose as a corbon source by converting it into
Xylulose-5- phosphate an intermediate in the
pentose phosphate pathway. The ara operon
codes for 3 enzymes .that are required to catalyze
the metabolism of arabinose encoded by 3 genes
araB,araA ,ara D.It is called ara BAD
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Structural genes

8. Control or regulatory genes
 The three structural genes in an operon is
regulated by the ara C gene product .
 The ara C gene is present near by operator gene
and transcribed from its own promoter which is
nearby ara O1 .
 The ara c is transcribed in opposite derection to
ara BAD gene the ara c produces repressor
molecules .
 Ara operon consists of two regulatory operator
site ,ara O1 and ara O2 .
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9. Mechanism of regulation of ara-operon
 The regulatory protein ara c ,encoded by ara c gene
mediates both possitive and negative control.
 Binding of a singnal molecule to the ara c protein
alters the conformational change from the repressor
form to an activator form.
 The ara c repressor protein regulates its own
synthesis ,by binding at ara O1.the ara c represses
its own transcription .
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10. Negative regulation
 Arabinose livels ara low and glucose levels are
high,ara c protein binds to the both ara I and ara O2
and these sites together to form a DNA loop of
about 210 base paris.
 The arabinose operon is switched off.
 These events block the access of RNA polymerase to
the PBAD promoter( which is a very weak promoter)
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11. Negative regulation structure
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12. Positive regulation
 The arabinose concentration is high or when
glucose concentration is very low ara c protein binds
to the arabinose ,changing its conformation leading
to the binding of ara c reprossor protein to the ara
O1 and ara I
 The DNA loop is opened and the transcription of
structural genes ara BAD mRNA synthesis enzymes
to metabolise arabinose
 The arabionse operon is switched on
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13. Structure of positive regulation
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14. Thank you