The P5 multicopy gene family in the MHC is related in sequence to human endogenous retroviruses HERV-L and HERV-16

P5 is believed to be a multicopy gene family with at least eight members re stricted to the major histocompatibility complex (MHC). Although the functi on of P5 genes is not known, one of the family members, P5-1, was found pre viously to be specifically transcribed in lymphoid cells and tissue. In thi s study, we used computer programs Censor and RepeatMasker, and dot plot an alysis to show that the major P5 family members are related in sequence to human endogenous retroviruses, HERV-L and HERV-16. The P5-HERV sequences ha ve at least 60% sequence identity with HERV-L within the pol region but dif fer significantly within the gag and LTR regions. The LTRs flanking the P5- HERV sequences share about 70% identity with the repeat element LTR16B. Str uctural analysis of open reading frames (ORFs) confirmed that the P5-1 cDNA is characterized by many stop codons and short putative coding regions res embling the patterns found in the HERV-L nucleotide sequence rather than th ose found in an mRNA sequence such as expressed by HLA class I genes. A 159 base pair (bp) ORF at the 5' end of the 2535 bp P5-1 mRNA may code for a p eptide of 52 amino acids with a domain identical in sequence to the signal peptide of HLA molecules. Furthermore, the P5-1 mRNA is complementary in se quence to retroviral pol mRNA. Therefore, the P5-1 genomic sequence appears to be an example of an HERV within the MHC that expresses an antisense tra nscript with a possible role in immunity to retrovirus infection.