Left Long Terminal Repeat Research Articles

A mutation in integrase can compensate for mutations in the simian immunodeficiency virus att site.

Sequences at the left terminus of U3 in the left long terminal repeat (LTR) and at the right terminus of U5 in the right LTR are important for integration of retroviral DNA. In the infectious pathogenic molecular clone of simian immunodeficiency virus strain mac239 (SIVmac239), 10 of the 12 terminal base pairs form an imperfect inverted repeat structure (5' TGGAAGGGATTT 3' [nucleotides 1 to 12] and 3' ACGATCCCTAAA 5' [nucleotides 10279 to 10268]). Nineteen different mutant forms of SIVmac239 proviral DNA with changes at one or more of the positions in each of the 12-terminal-base-pair regions were constructed. Viral replication was severely or completely compromised with nine of these mutants. Revertants appeared 40 to 50 days after transfection in two independent experiments with mutant 7, which contained changes of AGG to TAC at positions 5 to 7 in U3 and TCC to GAA at positions 10275 to 10273 in U5. Virus produced at these times from mutant 7 transfection replicated upon reinfection with only a slight delay when compared to the wild type. Sequence analysis of the LTR and integrase regions from infected cultures revealed two predominant changes: G to A at position 10275 in U5 and Glu to Lys at position 136 in integrase. Derivatives of clone 7 in which these changes were introduced individually and together were constructed by site-specific mutagenesis. Each change individually restored replication capacity only partially. However, the combination of both mutations restored replicative capacity to that of the original revertants. These results indicate that changes in integrase can compensate for mutations in the terminal nucleotides of the SIV LTR. The results further indicate that resistance to integrase inhibitors may include both integrase and LTR mutations.

Structural and functional analysis of some moulting hormone-responsive genes from Drosophila

Publisher Summary This chapter discusses steroid-regulated vertebrate genes. The most extensively studied steroid-responsive gene is the mouse mammary tumor virus (MMTV) provirus. In a variety of cell types, chromosomally inserted proviruses give rise to a single transcript, initiating within the left long terminal repeat (LTR). This transcript is present at very low levels until the cells are exposed to a glucocorticoid hormone. Within minutes following hormone treatment, the titre of the MMTV transcript rises, reaching a level that is at least 50-fold above the basal level within 2 hours. Glucocorticoid receptor binding sites are located within the MMTV LTR, and the same region of the LTR is both necessary and sufficient to confer glucocorticoid inducibility. Receptor binding sites are clustered between 80 and 300 base pairs upstream of the transcriptional initiation sites, and additional sites are present at several positions internal to the transcript. The chapter discusses ecdysteroid-responsive genes from drosophila and glue genes.

Long terminal repeats of endogenous mouse mammary tumour virus contain a long open reading frame which extends into adjacent sequences.

The integrated DNA copies (proviruses) of RNA tumour virus genomes are flanked by long terminal repeat (LTR) sequences, which are generated by the duplication of segments present at both ends of the viral RNA1. LTRs vary in length from 330 (ref. 2) to 1,328 (this paper) base pairs (bp). The primary viral transcript starts in the left LTR (L-LTR) and terminates in the right LTR (R-LTR)3. We have now sequenced both LTRs from an inherited (endogenous) mouse mammary tumour virus (MMTV) of the GR strain. This provirus, which is one of five present in the haploid genome of GR mice4, does not produce virus5 and is not identical with the exogenous MMTV of GR mice6. The provirus (GR40) could nevertheless be transcribed and its transcription hormonally regulated after cloning and transfection into mouse L cells7. The LTRs are identical and exhibit the following features: a terminal inverted 6-bp repeat, a Goldberg–Hogness (TATAAA) sequence, sequences which have been implicated in polyadenylation and the termination of transcription, and an open reading frame. The open reading frame of the R-LTR starts at the env/LTR junction and encodes a 36,700-molecular weight (Mr) protein. The open reading frame of the L-LTR extends into the adjacent mouse sequence. A host sequence of 6 bp at the site of integration into the mouse chromosome is directly repeated.