Retroviral gag gene amber codon suppression is caused by an intrinsic cis-acting component of the viral mRNA

Abstract

In some type C retroviruses, translation of the pol gene appears to require translational suppression of the proximal gag amber codon. To identify the region of the viral nucleic acid responsible for synthesis of the pol gene products, a 300-base-pair DNA fragment containing the stop codon from a type C murine virus (AK virus) was inserted into the Escherichia coli lacZ gene such that the translational reading frame was maintained.

Translational Suppression in Retroviral GENE Expression

“…Moreover, it could be shown that suppression of the Mo-MuLV UAG termination codon i n uitro was stimulated to the same extent by tRNA isolated from MuLV-infected or uninfected NIH13T3 cells (Feng et al, 1989a). Similarly, as noted above, Panganiban (1988) observed that i n uiuo suppression of the UAG codon a t the gag-pol junction occurred with the same efficiency in Mo-MuLV-infected and uninfected mouse cells. Taken together, these findings led Feng et al (1989a) to conclude that the glutamine suppressor tRNA occurs normally within the tRNA population of uninfected cells and is not altered or induced in response to virus infection.…”

“…These results provided direct evidence that the viral mRNA contains signals promoting suppression a t the gag-pol junction, and also implied that these signals do not extend beyond the limits of the restriction fragment used in Panganiban's experiments (Panganiban, 1988). In addition, because the constructs did not encode any viral proteins, and because essentially identical results were obtained in both virusinfected and uninfected cells (Panganiban, 19881, the data argue that the viral signals that result in suppression at the gag-pol junction are completely cis-acting.…”

“…The possibility that type C retroviral mRNA contains signals promoting suppression was tested directly in a series of experiments by Panganiban (1988). He isolated a restriction fragment of the MuLV genome containing 37 codons from the 3' end ofgag, the UAG codon a t the gag-pol junction, and 62 codons from the 5' end of pol.…”

“…3 probabiy do not exist in the viral mRNAs of other mammalian type C retroviruses that undergo readthrough suppression (Panganiban, 1988;ten Dam et al, 1990). Even more importantly, it has been pointed out (Panganiban, 1988) that a stem-loop structure containing the UAG codon is unlikely, because the structure would have to be disrupted before ribosome movement and misreading of the UAG codon could occur.…”

“…Presumably, there is a competition between suppressor tRNA and a release factor that determines whether suppression can take place. In addition, it has been proposed that suppression may be promoted by base pairing between one or two nucleotides immediately downstream of the termination codon in the message and the corresponding bases 5' of the anticodon in the tRNA (Engelberg-Kulka, 1981;Panganiban, 1988).…”