An avian leukosis virus subgroup J isolate with a Rous sarcoma virus-like 5′-LTR shows enhanced replication capability Gao, Yanni and Guan, Xiaolu and Liu, Yongzhen and Li, Xiaofei and Yun, Bingling and Qi, Xiaole and Wang, Yongqiang and Gao, Honglei and Cui, Hongyu and Liu, Changjun and Zhang, Yanping and Wang, Xiaomei and Gao, Yulong,, 96, 150-158 (2015), doi = https://doi.org/10.1099/vir.0.071290-0, publicationName = Microbiology Society, issn = 0022-1317, abstract= Avian leukosis virus subgroup J (ALV-J) was first isolated from meat-producing chickens that had developed myeloid leukosis. However, ALV-J infections associated with hemangiomas have occurred in egg-producing (layer) flocks in China. In this study, we identified an ALV-J layer isolate (HLJ13SH01) as a recombinant of ALV-J and a Rous sarcoma virus Schmidt-Ruppin B strain (RSV-SRB), which contained the RSV-SRB 5′-LTR and the other genes of ALV-J. Replication kinetic testing indicated that the HLJ13SH01 strain replicated faster than other ALV-J layer isolates in vitro. Sequence analysis indicated that the main difference between the two isolates was the 5′-LTR sequences, particularly the U3 sequences. A 19 nt insertion was uniquely found in the U3 region of the HLJ13SH01 strain. The results of a Dual-Glo luciferase assay revealed that the 19 nt insertion in the HLJ13SH01 strain increased the enhancer activity of the U3 region. Moreover, an additional CCAAT/enhancer element was found in the 19 nt insertion and the luciferase assay indicated that this element played a key role in increasing the enhancer activity of the 5′-U3 region. To confirm the potentiation effect of the 19 nt insertion and the CCAAT/enhancer element on virus replication, three infectious clones with 5′-U3 region variations were constructed and rescued. Replication kinetic testing of the rescued viruses demonstrated that the CCAAT/enhancer element in the 19 nt insertion enhanced the replication capacity of the ALV-J recombinant in vitro., language=, type=