Many enzymes in biochem pathways are in complex form, such as ATP synthase, some fatty acids synthase, enzyme complexes in electron transport chains. How do those proteins expressed in cells? Are the complex protein encoded by a single gene or an operon?
It varies. Some viruses encode all of their proteins as a single gene that is translated as a polyprotein that is cut apart by specific proteases encoded by the virus. The subunits of mitochondrial ATP synthase, which you mentioned, is encoded by a combination of nuclear and mitochondrial genes. In bacteria, it is common for many, if not all, of the structural and regulatory genes that are in a biochemical pathway to be encoded within an operon. Interestingly, though, they are not always on the same strand of DNA.
It varies. Some viruses encode all of their proteins as a single gene that is translated as a polyprotein that is cut apart by specific proteases encoded by the virus. The subunits of mitochondrial ATP synthase, which you mentioned, is encoded by a combination of nuclear and mitochondrial genes. In bacteria, it is common for many, if not all, of the structural and regulatory genes that are in a biochemical pathway to be encoded within an operon. Interestingly, though, they are not always on the same strand of DNA.
Adam's answer is cogent and addresses the question appropriately. As few additional examples, you can refer to the following papers:
1) The impact of horizontal gene transfer in shaping operons and protein interaction networks--direct evidence of preferential attachment.(http://www.ncbi.nlm.nih.gov/pubmed/18218112)
2) "Transcription of glycolytic genes and operons in Bacillus subtilis"(http://www.ncbi.nlm.nih.gov/pubmed/11489127)
All are possibilities because in developed organism more than one gene is involved in enzyme activity
All options are realized, as pointed out by Adam. The probably most abundant enyzme of all (up to 50% of protein of plant leaves!), Ribulose-1,5-bisphosphate carboxylase/oxygenase (or "Rubisco"), which fixates CO2 in plants, consists of a small subunit encoded by the nuclear genome and a large subunit encoded by the plastid (chloroplast) genome. Very finely regulated!
I agree with Adam's answer! Most other suggestions here are absolutely right! But the thing to remember is that sometimes different stimulus or triggers can influence a gene to transcribe for different mRNA's and hence different proteins. Regulation also occurs during splicing of the RNA as well.
Some plant-specific classical examples for phenylpropanoid pathway are here: http://onlinelibrary.wiley.com/doi/10.1034/j.1399-3054.1999.100119.x/epdf, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2860624/ .
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