Hi, I'm new to R and working through some existing code. I'm wanting to save a GRanges object to disk and then be able to reload it, preserving the metadata, so the object I restore to memory is the same as the one I saved to disk. The object I have in memory is as follows:

head(gr)

GRanges object with 6 ranges and 6 metadata columns:
                    seqnames      ranges strand |      TXID   TXSTART     TXEND
                       <Rle>   <IRanges>  <Rle> | <integer> <integer> <integer>
  ENST00000456328.2     chr1 11869-14409      + |         1     11869     14409
  ENST00000515242.2     chr1 11872-14412      + |         2     11872     14412
  ENST00000518655.2     chr1 11874-14409      + |         3     11874     14409
  ENST00000450305.2     chr1 12010-13670      + |         4     12010     13670
  ENST00000473358.1     chr1 29554-31097      + |         5     29554     31097
  ENST00000469289.1     chr1 30267-31109      + |         6     30267     31109
                               GENEID            TXNAME   TxWidth
                      <CharacterList>       <character> <integer>
  ENST00000456328.2 ENSG00000223972.4 ENST00000456328.2      1657
  ENST00000515242.2 ENSG00000223972.4 ENST00000515242.2      1653
  ENST00000518655.2 ENSG00000223972.4 ENST00000518655.2      1483
  ENST00000450305.2 ENSG00000223972.4 ENST00000450305.2       632
  ENST00000473358.1 ENSG00000243485.2 ENST00000473358.1       712
  ENST00000469289.1 ENSG00000243485.2 ENST00000469289.1       535
  -------
  seqinfo: 25 sequences (1 circular) from an unspecified genome; no seqlengths

I save it to a txt file as follows:

df = as(gr, "data.frame")
write.table(df, filename, sep = "\t", row.names = TRUE, col.names = TRUE, quote = FALSE)

I then read it back in as follows:

df2 = read.table(filename, header = TRUE, sep = "\t", dec = ".")
gr2 = makeGRangesFromDataFrame(df2, keep.extra.columns=TRUE)
head(gr2)

resulting in:

GRanges object with 6 ranges and 6 metadata columns:
                    seqnames      ranges strand |      TXID   TXSTART     TXEND
                       <Rle>   <IRanges>  <Rle> | <integer> <integer> <integer>
  ENST00000456328.2     chr1 11869-14409      + |         1     11869     14409
  ENST00000515242.2     chr1 11872-14412      + |         2     11872     14412
  ENST00000518655.2     chr1 11874-14409      + |         3     11874     14409
  ENST00000450305.2     chr1 12010-13670      + |         4     12010     13670
  ENST00000473358.1     chr1 29554-31097      + |         5     29554     31097
  ENST00000469289.1     chr1 30267-31109      + |         6     30267     31109
                               GENEID            TXNAME   TxWidth
                             <factor>          <factor> <integer>
  ENST00000456328.2 ENSG00000223972.4 ENST00000456328.2      1657
  ENST00000515242.2 ENSG00000223972.4 ENST00000515242.2      1653
  ENST00000518655.2 ENSG00000223972.4 ENST00000518655.2      1483
  ENST00000450305.2 ENSG00000223972.4 ENST00000450305.2       632
  ENST00000473358.1 ENSG00000243485.2 ENST00000473358.1       712
  ENST00000469289.1 ENSG00000243485.2 ENST00000469289.1       535
  -------
  seqinfo: 25 sequences from an unspecified genome; no seqlengths

GENEID and TXNAME are now listed as 'factor'. Am I saving and loading correctly? If so how do I convert GENEID to 'CharacterList' and TXNAME to 'character' ?

For TXNAME it seems I can use: gr2$TXNAME = as.character(gr2$TXNAME)
But typeof(gr$GENEID) says 'S4' and typeof(gr2$GENEID) says 'integer' so I have no idea how to handle this.

If I have restored the same object to memory I would expect GENEID to be listed as 'CharacterList' and typeof to say 'S4'

Thank you