DRT
Description
DRT stands for Defense-associated Reverse Transcriptases. DRTs are a widespread and highly diverse family of defense systems, characterized by reverse transcriptase (RTs) components with antiphage properties. These RTs belong to the so-called group of unknown RTs (UG) and are closely related to the Abortive Infection system (Abi) RTs.
DRT systems were experimentally validated in Escherichia coli and demonstrated to be effective against an array of diverse phages. So far, DRTs have been classified in 9 different types. Essential components of each DRT type are:
- DRT Type 1: UG1 (RT-nitrilaseTM domains)
- DRT Type 2: UG2 (RT domain)
- DRT Type 3: UG3 (RT domain), UG8 (RT domain) and ncRNA
- DRT Type 4: UG15 (RT-alphaRep domains)
- DRT Type 5: UG16 (RT domain)
- DRT Type 6: UG12 (RT-alphaRep domains)
- DRT Type 7: UG10 (PrimS-RT-alphaRep domains)
- DRT Type 8: UG7 (RT-alphaRep-PDDExK domains)
- DRT Type 9: UG28 (RT-alphaRep domains) and ncRNA
Molecular mechanism
The molecular mechanisms of DRTs are mostly unkown, except for DRT2 and DRT9.
DRT type 2
The defense mechanism DRT2 is composed of a RT gene and an upstream gene transcribed into a non-coding RNA (ncRNA). These genes are constitutively expressed and the ncRNA folds into a hairpin structure. The ncRNA is continuously reverse-transcribed by the RT in circles, producing a concatemeric single-stranded cDNA (ccDNA). Upon phage infection, the synthesis of the ccDNA and its complementary strand is increased. The DRT2 RT is able to switch from a ncRNA template to a ccDNA template resulting in the consecutive synthesis of single-stranded and double-stranded DNA by the same polymerase. Expression of this neo gene releases a neo protein that is highly toxic to the bacterium. Its production leads to rapid growth arrest and programmed dormancy.
Example of genomic structure
A total of 9 subsystems have been described for the DRT system.
Here are some examples found in the RefSeq database:
The DRT6 system in Polaribacter sp. L3A8 (GCF_009796785.1, NZ_CP047026) is composed of 1 protein: DRT6 (WP_158838312.1)
The DRT7 system in Bacillus thuringiensis (GCF_020809145.1, NZ_CP083117) is composed of 1 protein: DRT7 (WP_016090556.1)
The DRT8 system in Vibrio natriegens (GCF_001680065.1, NZ_CP016349) is composed of 2 proteins DRT8 (WP_065299195.1) DRT8b (WP_155759966.1)
The DRT9 system in Escherichia coli (GCF_004801575.1, NZ_CP039298) is composed of 1 protein: DRT9 (WP_211092143.1)
The DRT_1 system in Vibrio parahaemolyticus (GCF_013393865.1, NZ_CP040101) is composed of 2 proteins drt1a (WP_179000524.1) drt1b (WP_179000525.1)
The DRT_2 system in Chryseobacterium indoltheticum (GCF_003815915.1, NZ_CP033929) is composed of 1 protein: drt2 (WP_228421416.1)
The DRT_3 system in Enterobacter sp. JBIWA008 (GCF_019968765.1, NZ_CP074149) is composed of 2 proteins drt3a (WP_223562544.1) drt3b (WP_223562545.1)
The DRT_4 system in Vibrio diabolicus (GCF_002953355.1, NZ_CP014133) is composed of 1 protein: drt4 (WP_104973818.1)
The DRT_5 system in Klebsiella pasteurii (GCF_018139045.1, NZ_CP073236) is composed of 1 protein: drt5 (WP_211811523.1)