Table 1 Enzymes involved in mRNA degradation and modification during T4 infection.
Enzyme | Origin | Reaction catalyzed. Main properties | Role in T4 development |
|---|---|---|---|
RNase E | E. coli | Endonuclease. Produces 5'-P termini. Activated by 5'-monophosphorylated RNA. Scaffold of the degradosome | Major role in mRNA degradation throughout the phage developmental cycle. |
RNase G | E. coli | Endonuclease. Produces 5'-P termini. Activated by 5'- monophosphorylated RNA. | Cuts in the 5' regions of some early RegB processed transcripts. |
RegB | T4 | Sequence-specific endonuclease. Produces 5'-OH termini. Requires S1 r-protein as co-factor | Inactivates early transcripts by cleaving in Shine-Dalgarno sequences. Expedites early mRNA degradation. |
RNase LS | E. coli | Endonuclease. Its activity depends on rnlA and rnlB loci. Associated in a multiprotein compex. | Cleaves within T4 middle and late transcripts and expedites their degradation. |
RNase II RNase R Polynucleotide phosphorylase | E. coli | 3'-5' exonucleases. PNPase requires 3'-OH termini; the other two are indifferent to the nature of the 3' terminus. | Degrade mRNAs. The relative contribution of each RNase has not been determined. |
PrrC | E. coli | tRNAlys anticodon nuclease. Normally silent in E. coli but activated by the T4-encoded Stp polypeptide. | Deleterious to T4 propagation if Pnk or Rli1 enzymes are inactivated. |
Polynucleotide kinase (PNK) | T4 | Phosphorylation of 5'-OH polynucleotide termini. Hydrolysis of 3'-terminal phosphomonoesters and of 2',3'-cyclic phosphodiesters | Counteracts, together with T4 RNA ligase 1, host tRNA anticodon nuclease PrrC. Makes RegB-processed RNA substrates for RNases E and G. |
Dmd | T4 | An early product that binds the RnlA protein, a member of RNase LS | Antagonist of RNase LS |
Poly(A) polymerase | E. coli | Addition of poly(A) tails to the 3' end of RNAs | Probably inactivated after T4 infection |
RNA pyrophospho-hydrolase (RppH) | E. coli | Hydrolysis of a pyrophosphate moiety from the 5'-triphosphorylated primary transcripts. | Not yet investigated |