It is said that the structural genes are transcribed continuously. For example lets take the lac operon; if the gene for galactosidase is fully transcribed then transcription should end to synthesize the next gene, lets say galactosidase permease. But since all the genes are under single promoter it is also not possible.
On the other hand lets suggest that transcription occured continuously for all the genes. now, during translation how do ribosomes identify the last codon of first gene and cut the peptide sequence and start new protein synthesis in the next codon islet? If there is any stop codon then the ribosomes would dissociate from RNA and could not synthesize the upstream proteins.
HI Guruprasad
In prokaryotes for example like you mentione 'lac operon'....you are right that the genes under regulation of a single regulatory element are transcribed as one transcript (referred to as polycistronic transcript)....however the ribosomes do not cut inbetween the two proteins......it actually reconizes a ribosome binding site before the first AUG of each ORF and starts translation and onec the TERmination codon is encountered the ribosomal subunits release the protein and dissociates.....and they bind to next regions before the second ORF...
bye
Guruprasad, I'm not sure I'm following your question/ line of thought. Differences b/n prokaryotic and eukaryotic gene expression have been studied for almost 50 yrs and these differences are pretty much well understood. The prokaryotic genes are organized in polycistronic, operon like, structures and are transcribed in a single run. There are thousand studies and reviews explaining this in great details, as well as the difference b/n pro- and eukaryotic gene expression.
Usually after completing the peptide translation, protein sequence is not "cut" by the ribosome, but by other proteases, after recognizing specific linker sequences. You're right if there's a premature stop codon the ribosome will dissociate from the RNA and won't assemble downstream unless specific sequences recognized by the small ribosome subunit are present.
"How do ribosomes identify the last codon of first gene and cut the peptide sequence and start new protein synthesis in the next codon islet?" In prokaryotes, upstream of the next gene in the transcript there will be an internal ribosome entry site (IRES) that allow ribosomes to bind to the RNA and begin translation. In eukaryotes there is a similar concept, the Kozak sequence and 5' CAP, allow ribosomes to recognize and bind RNA upstream of the AUG. But IRES elements have been described in eukaryotic RNA.
Whoops, Mariano! In bacteria initiation is commonly in the middle of transcripts at ribosome binding sites (not IRESs). May seem like a semantic difference but still...it's good to keep these concepts separate.
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