A repressible operon system is that regulates genetic material which normally remains active/operational and enzymes formed by its structural genes present in the cell till the operon is switched off when concentration of an end product crosses a threshold value. Repressible operon system usually occurs in anabolic pathways, e.g., tryptophan operon, arginine operon. Each has the following parts:
(1) Structural Genes:
They are genes which take part in synthesis of polypeptide/proteins/enzymes through the formation of specific m RNAs. Tryptophan operon has five structural genes-E,D,C,B and k.
(2) Operator Gene:
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It provides passage to UN A polymerase moving from promoter to structural genes. Operator gene of repressible operon is normally kept swtiched on as aporepressor formed by regulator gene is unable to block the gene.
(3) Promoter Gene:
It is initiation/recognition point for RNA polymerase.
(4) Regulator Gene:
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The gene produces an apropressor.
(5) Aporepressor:
It is a proteinaceous substance formed through the activity of regulator gene. It is able to block operator gene only when a corepressor is also available.
(6) Corepressor:
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The nonproteinaceous component of repressor which can be end product (feed back inhibition/repression) of the reaction mediated through enzyme synthesized by structural genes. Corepressor of tryptophan operon is tryptophan. It combines with apropressor, forms repressor which then blocks the operator gene to switch off operon.
Multigene Families:
Multigene family is a group of similar or nearly similar genes that help in forming multiple copies either for meeting higher demand or time/tissue specific products. The former are called repeated genes. Globin gene family is an example of time/ tissue specific multigene family.
Repeated Genes:
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There are genes occurring in multiple copies or having variant repetition are called repeated genes, e.g., histone genes, t RNA genes, Ss RNA genes, actin genes, Sn RNA genes, heat shock protein genes.
Sn RNA:
Small nuclear RNA. Uridine rich, seven types, named U1 to U7. They are stable and ubiquitous. Each Sn RNA combines with 7-8 molecules of molecules of protein to form ribonucleoprotein or Sn RNP. Sn RNPs are involved in splicing (U1 and U2), r RNA processing (U3) and m RNA processing.