Cleavage sites of restriction endonucleases EcoRI, BamHI and HindIII on chicken embryo rDNA

Detection and mapping of homologous, repeated and amplified DNA sequences by DNA renaturation in agarose gels

A new molecular hybridization approach to the analysis of complex genomes has been developed. Tracer and driver DNAs were digested with the same restriction enzyme(s), and tracer DNA was labeled with 32P using T4 DNA polymerase. Tracer DNA was mixed with an excess amount of driver, and the mixture was electrophoresed in an agarose gel. Following electrophoresis, DNA was alkali-denatured in situ and allowed to reanneal in the gel, so that tracer DNA fragments could hybridize to the driver only when homologous driver DNA sequences were present at the same place in the gel, i.e. within a restriction fragment of the same size. After reannealing, unhybridized single-stranded DNA was digested in situ with S1 nuclease. The hybridized tracer DNA was detected by autoradiography. The general applicability of this technique was demonstrated in the following experiments. The common EcoRI restriction fragments were identified in the genomes of E. coli and four other species of bacteria. Two of these fragments are conserved in all Enterobacteriaceae. In other experiments, repeated EcoRI fragments of eukaryotic DNA were visualized as bands of various intensity after reassociation of a total genomic restriction digest in the gel. The situation of gene amplification was modeled by the addition of varying amounts of lambda phage DNA to eukaryotic DNA prior to restriction enzyme digestion. Restriction fragments of lambda DNA were detectable at a ratio of 15 copies per chicken genome and 30 copies per human genome. This approach was used to detect amplified DNA fragments in methotrexate (MTX)-resistant mouse cells and to identify commonly amplified fragments in two independently derived MTX-resistant lines.

Cloning of Xenopus laevis nuclear poly(A)-rich RNA sequences. Evidence for post-transcriptional control

cDNA/RNA hybridization experiments of polysomal and nuclear poly(A)-rich RNA from early tadpole stages of Xenopus laevis revealed that part of the nuclear poly(A)-rich RNA sequences are not present within the polysomal polyadenylated RNA. For a more detailed analysis of these sequences we have cloned double-stranded cDNA derived from tadpole nuclear poly(A)-rich RNA in the PstI cleavage site of pBR 322. By colony screening with 32P-labelled cDNA from polysomal and nuclear poly(A)-rich RNA of the tadpole stage we could identify and isolate some of the cloned sequences, which are present only within the nuclear RNA. However, hybridization with cDNA from polysomal poly(A)-rich RNA of the gastrula stage indicated that at least one of those sequences which are confined to the nucleus at tadpole stage may serve as mRNA at gastrula stage. We present evidence that nuclear and polysomal poly(A)-rich RNA molecules containing the same nucleotide sequence differ in size and that size reduction at the level of processing precedes and may enable cytoplasmic export. We conclude that, besides stage-specific regulation of transcription, post-transcriptional control mechanisms are also involved in gene expression during embryonic development.