Endogenous murine leukemia proviral long terminal repeats contain a unique 190-base-pair insert.

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Abstract
The nucleotide sequence in the U3-R regions of the long terminal repeat (LTR) associated with NFS-Th-1 xenotropic murine leukemia virus (MuLV) DNA and the LTR components of 5 endogenous proviruses cloned from BALB/c mouse chromosomal DNA were determined. The 5 endogenous MuLV LTR contained the regulatory signals thought to be important in viral transcription, such as TATAA and CCAAT-like boxes. A unique feature in 4 of the endogenous LTR was the presence of a highly conserved 190-base-pair (bp) insert bound by 6 bp direct repeats located 48 bp upstream from the C-C-A-A-T sequence. This segment was absent from LTR associated with ecotropic, xenotropic or mink cell focus-forming (MCF) MuLV proviruses. All 5 endogenous LTR segments also contained a 14 bp duplication of a sequence located near the 5'' end of the first component of the long (> 72 bp) direct repeat of ecotropic and MCF MuLV LTR. An evolutionary scheme relating LTR associated with endogenous MuLV proviral DNA to those found in ecotropic or xenotropic proviruses is presented. Nucleotide sequence analysis also suggested that the U3 region of the MCF247 MuLV LTR is derived from an NFS xenotropic related MuLV.