Hi , i am trying to rename the seqlevels of granges (feat_ranges) from type  I, II, III, ...,VII, Mito   to 1,2,3,...16, 17. Here are steps that i took but didn't worked for me. 

>library(GenomicFeatures); library("biomaRt"); library(systemPipeR);library(GenomeInfoDb)
>txdb <- makeTxDbFromBiomart(biomart = "ensembl", dataset = "scerevisiae_gene_ensembl")
>myfeatures <- c("tx_type", "promoter", "intron", "exon", "cds", "intergenic")
>feat_ranges <- genFeatures(txdb, featuretype=myfeatures, reduce_ranges=FALSE, upstream=1000, downstream=0)newStyle=mapSeqlevels(seqlevels(feat), as.character(c(1:17)))feat=renameSeqlevels(feat,newStyle)

>newStyle=mapSeqlevels(seqlevels(feat),as.character(c(1:16))) # did not worked for me

> feat_ranges

GRangesList object of length 12:
$protein_coding 
GRanges object with 6692 ranges and 3 metadata columns:
      seqnames         ranges strand   |      feature_by featuretype_id    featuretype
         <Rle>      <IRanges>  <Rle>   | <CharacterList>    <character>    <character>
             I [  335,   649]      +   |         YAL069W        YAL069W protein_coding
             I [  538,   792]      +   |       YAL068W-A      YAL068W-A protein_coding
             I [ 2480,  2707]      +   |       YAL067W-A      YAL067W-A protein_coding
             I [10091, 10399]      +   |         YAL066W        YAL066W protein_coding
             I [12046, 12426]      +   |       YAL064W-B      YAL064W-B protein_coding
  ...      ...            ...    ... ...             ...            ...            ...
          Mito [65770, 66174]      +   |           Q0182          Q0182 protein_coding
          Mito [73758, 74513]      +   |           Q0250          Q0250 protein_coding
          Mito [74495, 75984]      +   |           Q0255          Q0255 protein_coding
          Mito [79213, 80022]      +   |           Q0275          Q0275 protein_coding
          Mito [85554, 85709]      +   |           Q0297          Q0297 protein_coding

...
<11 more elements>
-------
seqinfo: 17 sequences (1 circular) from an unspecified genome

Hi

I might miss something here, but why not just:

> seqlevels(feat_ranges)
 [1] "I"    "II"   "III"  "IV"   "V"    "VI"   "VII"  "VIII" "IX"   "X"   
[11] "XI"   "XII"  "XIII" "XIV"  "XV"   "XVI"  "Mito"
>

> seqlevels(feat_ranges) <- as.character(c(1:17))

> feat_ranges
GRangesList object of length 12:
$protein_coding
GRanges object with 6692 ranges and 3 metadata columns:
      seqnames         ranges strand   |      feature_by featuretype_id
         <Rle>      <IRanges>  <Rle>   | <CharacterList>    <character>
             1 [  335,   649]      +   |         YAL069W        YAL069W
             1 [  538,   792]      +   |       YAL068W-A      YAL068W-A
             1 [ 2480,  2707]      +   |       YAL067W-A      YAL067W-A
             1 [10091, 10399]      +   |         YAL066W        YAL066W
             1 [12046, 12426]      +   |       YAL064W-B      YAL064W-B
  ...      ...            ...    ... ...             ...            ...
            17 [65770, 66174]      +   |           Q0182          Q0182
            17 [73758, 74513]      +   |           Q0250          Q0250
            17 [74495, 75984]      +   |           Q0255          Q0255
            17 [79213, 80022]      +   |           Q0275          Q0275
            17 [85554, 85709]      +   |           Q0297          Q0297
         featuretype
         <character>
      protein_coding
      protein_coding
      protein_coding
      protein_coding
      protein_coding
  ...            ...
      protein_coding
      protein_coding
      protein_coding
      protein_coding
      protein_coding

...
<11 more elements>
-------
seqinfo: 17 sequences (1 circular) from an unspecified genome
>

Regards, Hans-Rudolf