Synthesis of the tomato golden mosaic virus AL1, AL2, AL3 and AL4 proteins in vitro.

Transcripts derived from the leftward region of tomato golden mosaic virus DNA A were translated in wheat-germ and rabbit reticulocyte lysate systems. The largest protein (M(r) 40K) produced from transcripts encompassing open reading frame (ORF) AL1 was identified as the AL1 protein by immunoprecipitation with AL1-specific antibodies. However the main product was a protein of M(r) 10K, that was shown by in vitro mutagenesis to be the product of AL4, an ORF contained within AL1 DNA in a different reading frame. Translation of transcripts containing ORF AL2 or ORF AL3 gave the AL2 and AL3 proteins respectively; both proteins were also efficiently produced from transcripts containing both ORFs which overlap over about two-thirds of their length. Translation of a transcript containing the four ORFs gave all four proteins, consistent with a previous report that three of these (AL1, AL2 and AL3) can be translated from a single polycistronic RNA in transgenic tobacco plants. It is suggested that the leftward region of DNA A of the whitefly-transmitted geminiviruses may be expressed by two principal messenger RNAs, one encoding the AL1 and AL4 proteins and the other encoding the AL2 and AL3 proteins, and that the AL4 and AL3 proteins may be translated from the messenger RNAs by a leaky scanning mechanism.