Phosphorylase Nucleus

A discussion of all the enzymes known to be associated with the nucleus is unnecessary for the purposes of this text however, it might be valuable to review the nuclear association of some hydrolytic enzymes. The presence of adenosine triphosphatase in the nucleus remains controversial. Although some investigators found it in large amounts, none was present in the nuclear preparations of others but since dinitro-phenol, which activates mitochondrial adenosine triphosphatase, has no effect on the nuclear enzyme, it seems that nuclear adenosine triphosphatase is different from the mitochondrial enzyme. The association of phosphorylase with the nucleus is also puzzling it is not clear what part the enzyme plays in the synthesis or breakdown of nuclear glycogen. 

The fact that nuclei do contain enzymes likely to be involved in nucleoside metabolism (nucleoside phosphorylase, adenosine deaminase, and guanase), together with the fact that nucleoside phosphorylase catalyzes the formation of nicotinamide ribosides, leads the authors to point out the possible role of the nucleus in controlling the formation of di- and triphos-phopyridine nucleotide coenzymes. 

All eukaryotic nascent mRNAs, except for some eukaryotic viral RNAs, contain 5 -end cap structures. The cap consists of an N-methylguanine nucleotide connected to the S end (nucleotide N) of the RNA by three phosphate groups, i.e., m G(5 )ppp(5 )N h The cap is added to the free 5 ends of mRNAs in the nucleus before the polymerase has transcribed more than 20 nt. The biogenesis of the 5 -end cap involves a series of enzymatic reactions catalyzed by nucleotide phosphorylase (pppN ppN -I- Pj), RNA guanylyltransferase (ppN -I- GTP GpppN -I- PPj), and RNA methyltransferase (GpppN -P AdoMet - m GpppN -P AdoHcy). 

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