Problems replacing character values in a dataframe in R

One of my column in my datafra looks like this:

[1] "MIT""MIT""NUC""MIT""CYT""MIT""NUC""MIT""CYT""NUC""NUC""CYT""MIT""MIT""""""CYT" 

[18] "NUC""NUC""NUC""CYT""CYT""CYT""CYT""CYT""MIT""CYT""CYT""MIT""ME1""NUC""MIT""CYT""NUC" 

[35] "ME1""EXC""MIT""CYT""MIT""MIT""ME2""ME2""""""ME3""VAC""NUC""ME2""EXC""VAC""ME3"

I need to change "ME3" for ME.

When I used this code:

df_norm_pre$MIT["ME1"]="ME"

I find this error:

Error in $<-.data.frame(*tmp*, MIT, value = c("MIT", "MIT", "NUC", : replacement has 1550 rows, data has 1549.

Could someone help me with this problems?