Structural Biochemistry/The Signal Recognition Particle System in E. coli

The signal recognition particle (SRP) system, which contains the SRP and its receptor, plays a major role in targeting of ribosomes and IMP-encoding transcripts to the endoplasmic reticulum of eukaryotes and the cytoplasmic membrane of bacteria.

There are 3 essential components of the SRP system in E. coli:
 * The SRP protein – Ffh
 * The SRP receptor – FtsY
 * A small stable 4.5S RNA

The roles of SRP system in E. coli is related to the biosynthesis of IMPs:
 * SPR-mediated ribosome targeting (S-MRT)
 * SRP receptor-mediated ribosome targeting (SR-MRT)

1. S-MRT is based mainly on in vitro studies with purified components, has 2 stages: SRP binds to cytosolic mRNA-ribosome nascent chain complexes. The SRP-RNC complexes targeted to the membrane-bound FtsY.

2. SR-MRT is based mainly on in vivo studies, has 3 stages:

During FtsY translation, FtsY and ribosomes are targeted to the membrane in an SRP-independent manner. IMP-encoding mRNAs are targeted to the membrane-bound ribosomes. SRP interacts with a hydrophobic nascent polypeptide, the ribosomes, and the SRP receptor.

Both of the models require RNCs, SRP, and SRP receptor to form a complex on the membrane, transfer RNC to the translocon. However, SR-MRT differs from S-MRT by the additional targeting stages:

 Ribosome targeting: 

 mRNA targeting: 

IMP-encoding transcripts in E. coli are targeted to the membrane in a translation-independent manner.

The uracil-bias is an evolutionarily ancient feature of IMP-encoding mRNAs.

Uracil-richness is required for mRNA targeting to the membrane.