Well, the central dogma
demands that information does not
flow from protein
. This is, of course, not strictly true, as reverse transcriptase
has shown us. However, it is not necessary to do this to change the meaning of the message. The river of information can be diverted at several points - notably transcription
(DNA to RNA
) and translation (RNA to protein).
The first step (transcription) is dramatically altered by splicing in eukaryotes; this is where the introns are cut out of the message - parsing, basically. Since the spliceosome is a ensemble of proteins (a cellular machine), they are modifiable and therefore amenable to metabolic interference. More importantly, many human genes undergo alternative splicing - a cut 'n paste operation which allows several different mRNAs to be produced from one DNA sequence. Most notably, antibodies are 'shuffled' in this way to partially randomise the sequence without destroying the characteristic structure of the protein. In some organisms there is a bizarre practice of RNA editing where the mRNA is edited by enzymes. In trypanosome mitochondria, this takes the form of adding multiple Us - by another machine called the editosome imaginitive, eh?). An important part of the process is guide RNA (gRNA), which binds to the pre-edited mRNA.
Translation, on the other hand, has less messing about. Once all these '-osomes' have finished squabbling over the mRNA it is then faithfully translated, codon by codon, into an amino acid chain. However, the fun has only just started - the newly synthesised polypeptide is not yet a finished product. Many eukaryotic proteins are now modified in all sorts of ways. Posttranslational modifications include being cut in half, glycolylated, methylated and fitted with cofactors.
But : to what extent are all these alterations to the basic DNA message carried out by metabolism - rather than the genes that code for the editors? Clearly, the editing proteins themselves can be modified by other proteins, which have thier own genes and so on. The problem here is that the whole genome is involved (ultimately) in producing each new protein - transcription factor genes, rRNA genes, vault protein genes etc etc.
For information to be added without the involvement of a gene, ("information contained in metabolism" would require a transmissible protein or RNA 'factor' which could modify cell mRNA or cell protein and replicate. This is known as an RNA virus. :)