Translation of satellite tobacco necrosis virus RNA modified by (±)-r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene is inhibited in a wheat germ cell-free system

Abstract

It has been shown that (±)-r-7,t-8-dihydroxy-f-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) modification of rabbit globin mRNA results in inhibition of translational initiation. In order to explore the possibility that modification of the 5′ cap structure was responsible for this inhibition, the naturally non-capped mRNA from satellite tobacco necrosis virus (STNV) was reacted with BPDE and translated in a wheat germ cell-free system. The extent of modification was 1.3 and 2.9 BPDE residues/molecule. High performance liquid chromatography of the modified nucleosides from en-zymatically hydrolyzed STNV RNA revealed that >90°70 of the nucleoside adducts were substituted at the exocyctic amino group of guanosine. The translational ability of the lower and higher modified STNV, measured by incorporation of [¹⁴C]amino acids into acid-precipitable polypeptides is inhibited by 55% and 63%, respectively. Polyacrylamide gel electrophoretic analyses of the translation products indicate that predominantly full-length coat proteins are synthesized but with the carcinogen-modified STNV the amount is reduced. On the other hand, 80S initiation complex formation is not inhibited as measured by binding of the BPDE-modified STNV to ribosomes and followed by glycerol gradient centri-fugation. Under these conditions, aurintricarboxylic acid completely inhibits 80S initiation complex formation in the presence of either modified or native STNV. These results suggest that inhibition of in vitro translation of BPDE-modified STNV, in contrast to that of globin mRNA, is not at the level of initiation complex formation but possibly by premature termination of growing polypeptides.