Plasmid 1833: pRET.IS.IRES-EGFP N1  See scanned map. This is the
original RET-EGFP vector. The highest virus titer that can be
obtained with this construct is ~4x10^4 cfu/ml: the mRNA
instability signal seems to block transcription of the virus
genome at least to some extent. There are two versions, N1 and
N2. Version N1 contains two XbaI sites and can be used for
transfection after isolation of the XbaI-XbaI fragment. However,
N1 contains an additional cryptic poly A signal between EGFP and
GHpA, and its orientation is the same as that of virus
transcription (from right to left in the figure). This means
that N1 can not be used for the virus production: a full-length
virus RNA is not produced efficiently. In contrast, N2 does not
have such a cryptic poly A signal and is suitable for virus
production. However, N2 contains an additional XbaI site between
EGFP and GHpA, making it very difficult to isolate all the
essential components of the vector on a single XbaI-XbaI
fragment. Because of a weak NEO promoter,