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Biomedical subjects

S Muthukrishnan

Publications and source records attributed to S Muthukrishnan.

At least 55 records · Page 3Linked to original sources
PubMed1976

Mechanism of formation of reovirus mRNA 5'-terminal blocked and methylated sequence, m7GpppGmpC.

Blocked and methylated 5' termini of reovirus mRNA are formed by viral cores at an early stage of transcription. Cores incubated in a complete transcription reaction mixture for 30 s, or in a mixture lacking UTP and ATP for a longer time, synthesize the "cap" structure, m7GpppGmpC. The dinucleotide ppGpC functions as substrate for a core-associated guanylyltransferase and is converted to GpppGpC by addition of pG from GTP. For optimal conversion, both the diphosphate terminus and phosphodiester bond are required. pGpC is not a substrate, but pppGpC is utilized after removal of the gamma-phosphate by a core nucleotide phospohydrolase. Methyltransferases also present in cores transfer methyl groups sequentially from S-adenosylmethionine (AdoMet) to the N7-position of the 5'-terminal guanosine followed by the 2'-OH of the penultimate guanosine. GpppGpC is hydrolyzed by cores in the presence of pyrophosphate to ppGpC, the predominant 5'-terminal structure of reovirus mRNA made in the absence of S-adenosylmethionine. N7-methylation prevents pyrophosphorolysis of m7GpppGpC, which may explain the increased proportion of blocked, methylated 5' termini in viral mRNA synthesized in the presence of S-adenosylmethionine. On the basis of these findings, the following reaction series is proposed for the synthesis of reovirus mRNA caps. In the series, AdoHcy is the abbreviation for S-adenosylhomocysteine (see article)9

Edetic Acid↗
PubMed1976

A protein binding the methylated 5'-terminal sequence, m7GpppN, of eukaryotic messenger RNA.

Ribosomal salt washes of Artemia salina embryos contain a protein(s) that binds [3H]m7GpppGpC and [3H]m7GpppGmpC, as measured by retention on nitrocellulose membrane filters. These oligonucleotides correspond in structure to the methylated 5'-terminal sequences (caps) present in many eukaryotic mRNAs. The cap binding protein does not bind the unmethylated counterparts of caps, e.g., [32P]GpppGpCp, or a derivative of m7GpppGmpC containing ring-opened m7G. None of the purified initiation factors IF-MP, IF-M2A, IF-M2B, IF-M3, or IF-MI binds the m7G-containing oligonucleotides.

Base Sequence↗
PubMed1976

Dependence of translation on 5'-terminal methylation of mRNA.

Many eukaryotic cell and viral mRNAs contain 5'-terminal m7G(5')ppp(5')N. Methylation is important for the in vitro translation of vesicular stomatitis virus and reovirus mRNAs. Unmethylated viral mRNAs are methylated by cell-free extrascts of wheat germ and L cells to form 5'-terminal structures of the type, m7GpppN and m7GpppNm, respectively. Rabbit globin mRNA also contains 5'-terminal 7-methylguanosine (m7G). Removal of the m7G by beta-elimination decreases translation. The efficient binding of mRNA to ribosomes is dependent on the presence of 5'-terminal m7G.

Animals↗
PubMed1975

mRNA methylation and protein synthesis in extracts from embryos of brine shrimp, Artemia salina.

Cell-free protein-synthesizing extracts prepared from the brine shrimp, Artemia salina, translate methylated mRNAs. Reovirus unmethylated mRNA is inactive as a template when methylation is prevented by the inhibitor, S-adenosylhomocysteine. A salina mRNAs from both undeveloped and developed embryos contain 5'-terminal 7-methylguanosine in an inverted 5'-5' linkage through three phosphate groups to the rest of the polynucleotide chain. Removal of the 7-methylguanosine by beta elimination converts the mRNA from an active form to one inactive in protein synthesis in extracts of A. salina or wheat germ. Extracts of undeveloped and developed embryos methylate reovirus unmethylated mRNA at the 5' ends to form 5'-terminal structures of the type, m7G(5')ppp(5')G and m7G(5')ppp(5')Gm.

Animals↗
PubMed1975

5'-Terminal 7-methylguanosine in eukaryotic mRNA is required for translation.

Unmethylated reovirus and VSV mRNAs are specifically methylated to form 5'-terminal structures of the type, m-7-G(5')ppp(5')N by protein synthesising extracts prepared from wheat germ and mouse L cells. Reticulocyte mRNA also contains 5'-terminal m-7-G. MRNAs having 5'-terminal m-7-G stimulate protein synthesis in vitro. Removal of m-7-G by beta-elimination abolishes translation of the mRNAs.

Animals↗
PubMed1975

Ribosome binding to reovirus mRNA in protein synthesis requires 5' terminal 7-methylguanosine.

Purified reovirus synthesizes in vitro a mixture of mRNA molecules that contain 5' terminal structures of the type ppG..., GpppG..., and m7GpppGm, the relative amount of each depending upon the presence of S-adenosylmethionine (SAM) or S-adenosylhomocysteine (SAH) in the transcription incubation mixture. Reovirus mRNAs containing 5' termini of the type ppG... and GpppG... can be specifically modified by wheat germ extracts in the presence of SAM to yield the structure m7GpppG...(Muthukrishnan et al., 1975). The mRNA methylase activity is ribosome-independent and is recovered almost entirely in the high speed supernatant fraction of wheat germ extracts. Its activity is inhibited by aurintricarboxylic acid. Ribosome binding experiments with reovirus mRNA and wheat germ extract indicate that only those mRNA molecules containing 5' terminal m7G are capable of participating in the initiation of protein synthesis and subsequent polysome formation. mRNA with unmethylated 5' termini do not form a complex with 40S ribosomal subunits. Discrimination between unmethylated and methylated reovirus mRNA, active in protein synthesis, apparently occurs at or before the formation of 40S-mRNA complexes. T1 or pancreatic RNAase digestion of methylated mRNA bound to 80S ribosomes yields 5' terminal fragments of apparent chain lengths about 32 and 36 nucleotides, respectively. A portion of the T1 RNAase-resistant fragments rebinds to ribosomes to form a nuclease-resistant complex. In contrast, the shorter 5' terminal oligonucleotide m7GpppGmpCpUp(Np)3Gp derived by RNAase T1 digestion of purified reovirus mRNA does not bind to ribosomes. The results suggest that 5' terminal m7G may function as the primary recognition signal for ribosome binding and that wheat germ ribosomes bind very close to the 5' termini of some of the species of methylated reovirus mRNAs.

Aurintricarboxylic Acid↗
PubMed1975

5'-Terminal m-7G(5')ppp(5')G-m-p in vivo: identification in reovirus genome RNA.

Methylated reovirus mRNA was synthesized in vitro in the presence of S-adenosyl-L-[methyl-3H]-methionine. Viral genome double-stranded RNA that was uniformly labeled with 32-P was isolated from purified virions. The RNAs were mixed and their 5'-terminal structures compared by electrophoretic and chromatographic analyses after enzymatic digestion. Both the mRNA and the corresponding strand in the genome RNA contain m-7G(5')ppp(5')G-m-pCp, indicating that infected cells synthesize viral RNA with blocked, methylated 5' termini.

Base Sequence↗
PubMed1975

Studies on the integument of cestodes. VI. Histochemical studies on the fate of structural polysaccharide in the integument of gravid proglottides of Taenia hydatigena.

Histochemical study of the structural polysaccharide in the integument of gravid proglottides revealed that the main cuticle is only feebly positive to PAS reaction when compared with that of other regions of the cestode body. The significance of such change in the PAS reaction has been discussed. It was observed that there is no change in the acid mucopolysaccharide of the outer layer of the integument.

Aldehydes↗
PubMed1975

Studies on the integument of cestodes. II. The nature of protein component in the integument of gravid proglottides of Taenia hydatigena.

The main cuticle of the gravid proglottides shows differences in their chemical make up, in that there is a transformation of SH groups into -S-S bonding resulting in the refractivity of the cuticle. Another feature is the coexistence of dityrosine linkages with -S-S bondings. The mode of dityrosine formation from tyrosine residues is found to be possible by the presence of a peroxidase enzyme system in the cuticle. The evolutionary and physiological significance of the occurrence of -S-S bonding and dityrosine links have been discussed.

Animals↗
PubMed1972

Delta-aminolaevulinate dehydratase, the regulatory enzyme of the haem-biosynthetic pathway in Neurospora crassa.

The activity of delta-aminolaevulinate dehydratase is very low in the mould Neurospora crassa compared with the activities detected in bacterial and animal systems. The enzyme is inducible in iron-deficient cultures by addition of iron and is repressed by protoporphyrin. The properties of the purified enzyme indicate its allosteric nature and susceptibility to feedback inhibition by coproporphyrinogen III. Neurospora extracts also contain a protein inhibitor of the enzyme and a small-molecule activator, which appears to be associated with the enzyme. The regulatory function of this enzyme in vivo is correlated with the accumulation of delta-aminolaevulinic acid in normal cultures of N. crassa. The decay curve of the iron-induced enzyme in vivo shows a biphasic pattern, with one of the components showing a half-life of 4-5 min.

Adenosine Diphosphate↗