When are parts of prokaryotic operons seperated?

[poobear]

So I've read that prokaryotes often have operons - they have polycistronic sites that are transcribed as 1 unit.
Though I havnt read that prokaryotes would have splicing as eukaryotes have in posttranscriptional processing.

So, when are they seperated?
Are the mRNA actually spliced? Or does the operon have one Shine-Delgarno sequence for each gene sequence in the mRNA, so that the ribosome can bind on more than 1 place on the mRNA? Or is it translated also as 1 unit and then cut of in posttranslational processing?

/ Thanks

[MrMistery]

generally there is one Shine-Dalgarno(watch the spelling) sequence for every gene

[beautifulmind420]

Form my understanding, polycistronic sites means 1 mRNA is responsible for production of more than one proteins. I know from one experiment that if you have nonsense mutation in lacZ gene, it would result in truncation in lacZ protein and lacY and lacA proteins wouldn't be produce at all.

[MrMistery]

would you mind providing a reference for that? i don't think that's the way it happens

[beautifulmind420]

Textbook: iGenetics by Peter J. Russell

[MrMistery]

OK, so I looked it up.
Most polycistronic mRNAs indeed have one Ribosome binding site for each gene(Watson et al. Molecular biology of the gene 6th edition. page 460). However, this is not always true.
sometimes an ORF(open reading frame) lacks a ribosome binding site and is only actively transcribed by virtue of the being downstream of a gene with a strong RBS. in these cases, the start codon of the downstream ORF often overlaps the 3' end of the upstream ORF(most often as the sequence 5'AUGA3' which contains a start and a stop codon). thus a ribosome that has just completed translating the upstream ORF is well positioned to begin translating from the start codon for the downstream ORF. this arrangement circumvents the need to a RBS to recruit the ribosome. This is known as translational coupling. (from Watson et al. Molecular Biology of the gene 6th edition. page 460). it may be translational coupling that happens in the lac operon.
It is important to note though, that translational coupling is the exception, not the rule.