Table 1.

Common and unique characteristics of the best known non-LTR retrotransposons in which the enzymatic activities have been experimentally reported. ORFs (number of Open Reading Frames), AP (AP-endonuclease), RT (Reverse Transcriptase), RH (RNase H), NAC (Nucleic Acid Chaperone), ORF1p protein with NAC activity (the human ORF1 bears CCHC domain and the mouse ORF1p protein lacks zinc fingers), DBD (DNA-Binding Domain at N- and C-terminal), EN (Endonuclease from R2 element, similar to type II restriction enzymes).

TE	Type	Size (kb)	Transcription (Mediated by)	ORFs	Domains	Unique Features
L1Tc (T. cruzi)	LINE (autonomous)	4.9	Pr77 internal promoter sequence (RNAPII-dependent)	1	AP, RT, RH, NAC	• Pr77-dual system: promoter (DPE motif) and HDV-like ribozyme. • 2A-self cleaving sequence. • RNaseH domain. • No antisense promoter activity.
human L1 (Homo sapiens)	LINE1 (autonomous)	6	L1 internal promoter located at 5’UTR of the element (RNAPII-dependent).	2	ORF1p, AP, RT	• AP domain shows 3’→5’ exonuclease proofreading activity. • Antisense promoter activity that regulates L1 mobilization.
mouse L1 (Mus musculus)	LINE1 (autonomous)	7	Repeating promoter motif located at 5’ UTR of the element (RNAPII-dependent) and followed by 5’UTR.	2	ORF1p, AP, RT	• Translation via dicistronic mRNA that contains an IRES. • Antisense promoter activity that regulates L1 mobilization.
R2Bm (Bombyx mori)	Site-specific (R2)	4.2	The R2 RNA is initially co-transcribed with host ribosomal 28S RNA by RNAPI.	1	N-DBD, RT, EN, C-DBD	• HDV-like ribozyme. • 5’ and 3’ regions of the R2 RNA are binding sites for R2 polyprotein.