INSECT IRIDESCENT VIRUS TYPE-6 ENCODES A POLYPEPTIDE RELATED TO THE LARGEST SUBUNIT OF EUKARYOTIC RNA-POLYMERASE-II

Cytoplasmic DNA viruses encode a DNA-dependent RNA polymerase (DdRP) t hat is essential for transcription of viral genes. The amino acid sequ ences of known large subunits of DdRPs contain highly conserved region s. Oligonucleotide primers, deduced from two conserved domains [RQP(T/ S)LH and NADFDG-DE] were used in PCR experiments for the detection of the corresponding gene of the genome of insect iridescent virus type 6 , also known as Chilo iridescent virus (CIV). A specific DNA product o f about 150 bp could be amplified and was used as a hybridization prob e against the CIV gene library to identify the corresponding gene. The gene encoding the DdRP was identified within the EcoRI fragments M (7 099 bp) and L (7400 bp) of CIV DNA, between map units 0.310 and 0.347 (7990 bp). The DNA nucleotide sequence (3153 bp) of the gene encoding the largest subunit of DdRP (RPO1) was determined. Northern blot hybri dization revealed the presence of a 3.4 kb RNA transcript in CIV-infec ted cells that hybridized to the CIV DdRP gene. This predicted viral p rotein consists of 1051 amino acid residues (120K) and showed consider ably higher similarity to the largest subunit of eukaryotic RNA polyme rase II than to the homologous proteins of vaccinia virus and African swine fever virus. Phylogenetic analysis suggested that the putative R PO1 of CIV could have evolved from RNA polymerase II after the diverge nce of the three types of eukaryotic RNA polymerases. The putative RPO 1 of CIV lacked the C-terminal domain that is conserved in eukaryotic, eubacterial and other viral RNA polymerases and in this respect was a nalogous to the RNA polymerases of Archaea. It is hypothesized that th e equivalent of the C-terminal domain may reside in another subunit of CIV DdRP encoded by an unidentified viral gene.