The chromosomal genes for black widow spider neurotoxins do not contain introns

The overlapping fragments of the chromosomal DNA from black widow spider Latrodectus mactans carrying genes for high-molecular-mass protein neurotoxins, alpha- and delta-latroinsectotoxins (alpha-LIT and delta-LIT) and alpha-latrotoxin (alpha-LTX), were PCR-amplified and cloned. Restriction analysis of the PCR products showed that the distribution and sizes of the restriction fragments coincided with those deduced from the earlier sequencing of cDNAs of the corresponding genes. It thus followed that the alpha-LIT and delta-LIT genes are intronless. Along with our data on the structure of the alpha-latrocrustotoxin (alpha-LCT), this implies that the lack of introns is a common feature of the black widow spider genes encoding high molecular mass neurotoxins.     

Two 16S-23S ribosomal DNA intergenic regions in different Treponema pallidum subspecies contain tRNA genes

Abstract The 16S-23S intergenic spacers of Treponema pallidum subspecies pallidum , Nichols strain, and Treponema pallidum subspecies pertenue , Gauthier strain, have been cloned, characterized and sequenced. Isoleucine and alanine tRNA genes have been identified within the 16S-23S intergenic regions on separate alleles of 293 and 303 bases, respectively. The two alleles are present in both T.p. pallidum and T.p. pertenue , and show no sequence differences between the bacterial subspecies. The ile-tRNA and ala-tRNA genes show 65% and 84% sequence identity, respectively, with the homologous genes of the related spirochete, Borrelia burgdorferi .     

Origin and evolution of group I introns in cyanobacterial tRNA genes.

Many tRNA(Leu)UAA genes from plastids contain a group I intron. An intron is also inserted in the same gene at the same position in cyanobacteria, the bacterial progenitors of plastids, suggesting an ancient bacterial origin for this intron. A group I intron has also been found in the tRNA(fMet) gene of some cyanobacteria but not in plastids, suggesting a more recent origin for this intron. In this study, we investigate the phylogenetic distributions of the two introns among cyanobacteria, from the earliest branching to the more derived species. The phylogenetic distribution of the tRNA(Leu)UAA intron follows the clustering of rRNA sequences, being either absent or present in clades of closely related species, with only one exception in the Pseudanabaena group. Our data support the notion that the tRNA(Leu)UAA intron was inherited by cyanobacteria and plastids through a common ancestor. Conversely, the tRNA(fMet) intron has a sporadic distribution, implying that many gains and losses occurred during cyanobacterial evolution. Interestingly, a phylogenetic tree inferred from intronic sequences clearly separates the different tRNA introns, suggesting that each family has its own evolutionary history.     

Analysis of a drosophila tRNA gene cluster: two tRNALeu genes contain intervening sequences

A recombinant DNA phage containing a cluster of Drosophila melanogaster tRNA genes has been isolated and analyzed. The insert of this phage has been mapped by in situ hybridization to chromosomal region 50AB, a known tRNA site. Nucleotide sequencing of the entire Drosophila tRNA coding region reveals seven tRNA genes spanning 2.5 kb of chromosomal DNA. This cluster is separated from other tRNA regions on the chromosome by at least 2.7 kb on one side, and 9.6 kb on the other. Two tRNA genes are nearly identical and contain intervening sequences of length 38 and 45 bases, respectively, in the anticodon loop. These two genes are assigned to be tRNALeu genes because of significant sequence homology with yeast tRNA3Leu, and secondary structure homology with yeast tRNA3Leu intervening sequence. In addition, an 8 base sequence (AAAAUCUU) is conserved in the same location in the intervening sequences of Drosophila tRNALeu genes and a yeast tRNA3Leu gene. Similar sequenes occur in all other tRNAs containing intervening sequences. The remaining five genes are identical tRNAIle genes, which are also identical to a tRNAIle gene from chromosomal region 42A. The 5' flanking regions are only weakly homologous, but each set of isoacceptors contains short regions of strong homology approximately 20 nucleotides preceding the tRNA coding sequences: GCNTTTTG preceding tRNAIle genes; and GANTTTGG preceding tRNALeu genes. The genes are irregularly distributed on both DNA strands; spacing regions are divergent in sequence and length.     

Two different snoRNAs are encoded in introns of amphibian and human L1 ribosomal protein genes.

We previously reported that the third intron of the X.laevis L1 ribosomal protein gene encodes for a snoRNA called U16. Here we show that four different introns of the same gene contain another previously uncharacterized snoRNA (U18) which is associated with fibrillarin in the nucleolus and which originates by processing of the pre-mRNA. The pathway of U18 RNA release from the pre-mRNA is the same as the one described for U16: primary endonucleolytic cleavages upstream and downstream of the U18 coding region produce a pre-U18 RNA which is subsequently trimmed to the mature form. Both the gene organization and processing of U18 are conserved in the corresponding genes of X.tropicalis and H.sapiens. The L1 gene thus has a composite structure, highly conserved in evolution, in which sequences coding for a ribosomal protein are intermingled with sequences coding for two different snoRNAs. The nucleolar localization of these different components suggests some common function on ribosome biosynthesis.     

Variations of the length of exons and introns in human genome genes

The exon-intron organization of genes of all human chromosomes was studied in relation to the density of gene distribution on DNA strands and on the number of introns in genes. The lengths of exons, introns and genes have been found to vary correlatively, and this correlation depends on the density of genes in human chromosomes. It has been established that genes with the exon-intron organization have similar tendencies of variation of the lengths of exons and introns with an increase in the number of introns.     

Most human carcinomas of the exocrine pancreas contain mutant c-K-ras genes

Using in vitro gene amplification by the polymerase chain reaction (PCR) and mutation detection by the RNAase A mismatch cleavage method, we have examined c-K-ras genes in human pancreatic carcinomas. We used frozen tumor specimens and single 5 micron sections from formalin-fixed, paraffin-embedded tumor tissue surgically removed or obtained at autopsy. Twenty-one out of 22 carcinomas of the exocrine pancreas contained c-K-ras genes with mutations at codon 12. In seven cases tested, the mutation was present in both primary tumors and their corresponding metastases. No mutations were detected in normal tissue from the same cancer patients or in five gall bladder carcinomas. We conclude from these results that c-K-ras somatic mutational activation is a critical event in the oncogenesis of most, if not all, human cancers of the exocrine pancreas.     

Two separate tyrosine protein kinases in human platelets

Tyrosine protein kinase activities were detected in the cytosolic fraction (PC-TPK) and the particulate fraction (PM-TPK) in human platelets using the synthetic peptide, E11G1 (Glu-Asp-Ala-Glu-Tyr-Ala-Ala-Arg-Arg-Arg-Gly) as a substrate. PC-TPK and PM-TPK were different in substrate specificities, divalent cation requirements and apparent Mr values. These results strongly suggest that in platelets there exist at least two separate tyrosine protein kinases; one is present in cytosol and the other might be associated with membranes.     

Amber, ochre and opal suppressor tRNA genes derived from a human serine tRNA gene.

Amber, ochre and opal suppressor tRNA genes have been generated by using oligonucleotide directed site-specific mutagenesis to change one or two nucleotides in a human serine tRNA gene. The amber and ochre suppressor (Su+) tRNA genes are efficiently expressed in CV-1 cells when introduced as part of a SV40 recombinant. The expressed amber and ochre Su+ tRNAs are functional as suppressors as demonstrated by readthrough of the amber codon which terminates the NS1 gene of an influenza virus or the ochre codon which terminates the hexon gene of adenovirus, respectively. Interestingly, several attempts to obtain the equivalent virus stock of an SV40 recombinant containing the opal suppressor tRNA gene yielded virus lacking the opal suppressor tRNA gene. This suggests that expression of an efficient opal suppressor derived from a human serine tRNA gene is highly detrimental to either cellular or viral processes.     

A statistical analysis of nucleotide sequences of introns and exons in human genes

DNA sequences of 56 human genes for which information on both exons and introns was available were examined. The variance in G+C content among genes is estimated and shown to be substantial. There is a high correlation in G+C content between exons and introns within the same gene. The dinucleotide frequencies of introns are similar to those of intergenic spacer regions and are in reasonable agreement with predictions from substitution rates estimated from pseudogenes, except that the observed deficiency of TA doublets is not predicted. Duplicated bases also show a frequency greater than the expectation under independence. There is marked variability among genes in the frequency of the doublet CG relative to its expectation under independence. This variation is evolutionarily conserved and is correlated with the G+C content. Pseudogenes behave as if they are in a low -G+C, CG-deficient part of the genome, although the genes from which they arose are variable in these respects.