The A312L 5′-UTR of Chlorella virus PBCV-1 is a translational enhancer in Arabidopsis thaliana

PBCV-1 (Paramecium bursaria Chlorella virus) is a large double stranded DNA virus that replicates in certain eukaryotic chlorella like green algae. The PBCV-1 A312L gene encodes a 33-kDa protein whose function currently is unknown. The 5′-UTR of the A312L mRNA is 153 nucleotides, longer than the 5′-UTR in any other PBCV-1 gene. The sequence 5′-AAAC was repeated 17 times within 156 bp 5′ to the A312L gene start codon and this sequence was repeated 13 times continuously in the 5′-UTR of the mRNA. Recombinant genes were constructed in vector pBI121 that contained the A312L 5′-UTR, in both the forward and inverse-complement orientations, fused to the GUS gene under the control of the CaMV 35S promoter. These constructs were introduced into Arabidopsis thaliana and the results indicated that the A312L 5′-UTR functions as a translational enhancer only in the forward orientation. Overall, the ratio of GUS enzyme activity to GUS mRNA was 15-fold higher in constructs derived from the A312L 5′-UTR in the forward orientation as compared to constructs containing the 5′-UTR in the inverse-complement orientation or those lacking the A312L 5′-UTR.