Initiation of translation by non-AUG codons in human T-cell lymphotropic virus type I mRNA encoding both Rex and Tax regulatory proteins

Human T-cell lymphotropic virus type I(HTLV-I) double-spliced mRNA exhibits two GUG and two CUG codons upstream to, and in frame with, the sequences e ncoding Rex and Tax regulatory proteins, respectively. To verify whether th ese GUG and CUG codons could be used as additional initiation codons of tra nslation, two chimeric constructs were built for directing the synthesis of either Rex-CAT or Tax-CAT fusion proteins. In both cases, the CAT reporter sequence was inserted after the Tax AUG codon and in frame with either the Rex or Tax AUG codon, Under transient expression of these constructs, othe r proteins of higher molecular mass were synthesized in addition to the exp ected Rex-CAT and Tax-CAT proteins. The potential non-AUG initiation codons were exchanged for either an AUG codon or a non-initiation codon, This all owed us to demonstrate that the two GUG codons in frame with the Rex coding sequence, and only the second CUG in frame with the Tax coding sequence, w ere used as additional initiation codons, In HTLV-I infected cells, two Rex and one Tax additional proteins were detected that exhibited molecular mas s compatible with the use of the two GUG and the second CUG as additional i nitiation codons of translation. Comparison of the HTLV-I proviral DNA sequ ence with that of other HTLV-related retroviruses revealed a striking conse rvation of the three non-AUG initiation codons, strongly suggesting their u se for the synthesis of additional Rex and Tax proteins.