Endogenous avian retroviruses contain deficient promoter and leader sequences

Functional analysis of the transcription control region located within the avian retroviral long terminal repeat

We used several quantitative assays of in vivo transient gene expression to dissect the elements within the Rous sarcoma virus long terminal repeat (LTR) which constitute the retroviral transcription control region. Site-directed deletion mutagenesis was used to locate and define the enhancer and promoter elements within the LTR. In addition, we inserted exogenous DNA fragments into the LTR to examine the effects of position and sequence on the activity of these LTR transcriptional elements. The Rous sarcoma virus enhancer element, which we propose is located entirely within the LTR, was shown to activate both the beta-globin and retroviral LTR promoters when located in cis. We observed a striking correlation between the degree of activation and the distance between the retroviral promoter and enhancer elements. The LTR promoter element mediated the activation effect of the enhancer element, as LTR deletion mutants containing only the enhancer and TATA box region expressed little activity. The promoter region encoded a low but significant level of transcriptional activity even in the absence of an enhancer. Overall LTR transcriptional activity declined sharply with increasing distance between the LTR promoter and initiator elements. These results shed light on both the importance of the spatial arrangement of the sequence elements within this eucaryotic transcription control region and on the functional interrelationship between these elements.

Transcriptional activity of avian retroviral long terminal repeats directly correlates with enhancer activity

Retroviral long terminal repeats (LTRs) contain elements responsible for the control of proviral transcription and gene expression. Molecular clones of the LTR region of a number of avian retroviruses have been isolated, and DNA sequence analysis of these clones reveals the existence of a related, but heterogeneous, family of LTRs. To examine the functional significance of the observed sequence differences, we have directly tested the abilities of several different avian retrovirus LTRs to act as promoters and enhancers of mRNA transcription. Our results indicate that large differences in LTR transcriptional activity exist and that these differences in gene expression directly correlate with LTR enhancer activity. In particular, we show that the LTR of Fujinami sarcoma virus is intermediate in both transcriptional and enhancer activity when compared with the very active LTRs of the exogenous viruses RAV-2 and Schmidt-Ruppin B and the much less active LTRs of the endogenous virus RAV-0 and its provirus ev-2. These results suggest that LTR enhancer activity may be the primary determinant of avian retroviral LTR transcriptional activity and, hence, oncogenic potential.

Bacterial beta-galactosidase as a marker of Rous sarcoma virus gene expression and replication

We have developed a convenient and sensitive assay of eucaryotic gene expression which uses the Escherichia coli lacZ gene product, beta-galactosidase, as a nonselectable marker. This system has been applied to the analysis of Rous sarcoma virus replication and gene expression. Avian cells were transfected with plasmids encoding in-frame gene fusions of the N-terminal portion of the gag gene to a 'lacZ gene, which requires both transcriptional and translational initiation signals; these were supplied by the virus long terminal repeat and leader region. Readily detectable quantities of beta-galactosidase were synthesized in transfected cells; it was demonstrated that the levels of enzyme activity induced in such cultures increased linearly with the input DNA concentration and also correlated with mRNA levels. By using a Rous sarcoma virus-derived vector containing the src gene and a related virus as a helper, it was shown that lac sequences were compatible with all phases of the virus life cycle. gag-lacZ fusion proteins were immunoprecipitable from cultures which stably expressed lacZ as well as src. Virus rescued from stably transfected cultures resulted in continued lac and src expression in recipient cells. One particular construction was efficiently transmitted as virus, although it lacked sequences thought to be important for encapsidation of RNA into virions. The data presented here demonstrate the use of lacZ as a marker of retrovirus gene expression and replication.

Molecular basis of host range variation in avian retroviruses

Previous genetic analysis has localized the region of the Rous sarcoma virus (RSV) env gene responsible for host range specificity to that encoding the middle one-third of gp85. To better understand the host range determinants, the relevant regions of the genomes of infectious molecular clones of the transformation-defective Prague strain of RSV, subgroup B (Pr-RSV-B) and Rous-associated virus 0 (RAV-0) (subgroup E) were sequenced and compared with the sequence of Pr-RSV-C. This comparative analysis identified two variable regions of low amino acid sequence homology flanked by highly conserved amino acid sequences. The first variable region (hr1) begins at base 5654 in the Pr-RSV-C sequence and encodes 32 amino acids. The second variable region (hr2) begins at base 5846 and encodes 27 amino acids. To test the role of the variable regions in host range specificity, we determined the sequence of this region of the env gene of NTRE-4, a recombinant virus between Pr-RSV-B and RAV-0 which exhibits an extended host range. This analysis revealed that the recombinant subgroup-encoding region of NTRE-4 is composed of 200 bases of RAV-0 sequence, including hr2, flanked by sequences which are otherwise of Pr-RSV-B origin. This study indicates that hr1 and hr2 are the domains of gp85 responsible for host range determination in avian retroviruses.

Transcriptional interference in avian retroviruses--implications for the promoter insertion model of leukaemogenesis

The downstream (3') long terminal repeat (LTR) of an avian retroviral provirus is unable to act as an efficient promoter of transcription when a transcriptionally active upstream (5') LTR is present. This transcriptional interference may explain the observation that only deleted proviruses have been observed inserted adjacent to c-myc in avian leukosis virus induced lymphomas of chickens.