No abstract

No abstract

Clones homologous to the 5' region of the Ultrabithorax gene of Drosophda melanogaster have been isolated from D. pseudoobscura, D. funebris and Musca domestica. Regions that encode most of the Ubx protein have been sequenced in all three of these species, and the 5' upstream region has been sequenced in D. funebris to a point -1000 bases upstream of the probable mRNA start site. Here we compare these sequences with those described elsewhere for D. melanogaster. Deduced amino acid sequences of the Ubx protein show 8% (D. pseudoobscura), 15% (D. funebris) and 22% (M. domestica) divergence from D. melanogaster. However, these figures mask very different rates of evolution in different regions of the protein. A glycine-rich ('hinge') region is conserved in each of these species, although its length is variable. Comparison of D. funebris and D. melanogaster sequences in the long 5' untranslated leader region of the mRNA, and in the region immediately upstream of the start point of transcription, reveals tightly conserved elements embedded in an otherwise non-homologous sequence. These conserved elements include a 118-bp region that spans the mRNA start site, an internally repetitive (TAA), region in the untranslated leader and a short repeated motif immediately upstream of the ATG codon that initiates the major open reading frame of the Ubx protein. Two other conserved elements were identified upstream of the transcription start site; both elements have structural features consistent with a role as recognition sites for regulatory proteins.

A chicken 3-tubulin cDNA probe has been used to screen two independently generated human genomic libraries. Of 13 EcoRI fragments detectable in a human genomic Southern blot experiment, 7 correspond in size to EcoRI fragments isolated from recombinant bacteriophage. The location of (8-tubulin-specific regions and the direction of transcription were determined within each cloned fragment. One clone (503) contained a (-tubulin-specific region of6.8 kilobase pairs (kbp) that included three intervening sequences as well as a number of inverted repeat structures. The remaining clones contained (3-tubulin-specific sequences that were close to or, in two cases, substantially less than 1.9 kbp long. Because mature human .0-tubulin mRNA is approximately 1.9 kbp long, these short DNA regions cannot on their own encode a functional (3-tubulin mRNA. Analysis using 3'-and 5'-specific probes derived from the chicken cDNA clone showed the presence ofboth ofthese end regions within one truncated tubulinlike sequence. A second short tubulin-specific region failed to hybridize with a 3'-specific probe. These short sequences are therefore likely to be examples of pseudogenes that have arisen by loss of a portion of DNA essential to the production of functional human f-tubulin mRNA.Microtubules are constituent parts of a diverse variety of eukaryotic subcellular structures. An integral part of the mitotic apparatus, cilia, flagella, and elements ofthe cytoskeleton, they consist principally of two soluble proteins, named a-and ,B-tubulin, each with a molecular weight ofabout 55,000. Work performed in this laboratory (1) has demonstrated that, in chickens, separate mRNAs encode a-and ,B-tubulin; thus,-these proteins are transcribed from distinct genes. Construction of cDNA clones from isolated chicken a-and /&tubulin mRNAs and use of these probes in Southern blotting experiments of whole genomic DNAs (2) suggested the presence of about 4 copies each per genome of a-and /3-tubulin genes in the chicken; the corresponding value in experiments using human DNA is about 14 copies per genome.The expression of a number of tubulin genes might account for the reported appearance of multiple a-and ,B-tubulins isolated from various sources (3-6). Alternatively, such heterogeneity may reflect posttranslational modifications of a protein transcribed from one or a few genes. The extent to which protein heterogeneity is reflected at the genetic level can only be adequately revealed by a direct examination of the structure and organization ofthe genes themselves. In addition, such an analysis is a necessary prerequisite to addressing questions relating to tubulin gene expression. Several lines of evidence-including protein sequence data (7), blot hybridization experiments (2), and the ability of a-and /-tubulins from different species to copolymerize (8)-strongly suggest that tubulins are highly evolutionarily conserved. This paper describes the isolation and characterization of cloned DNA fragments containing 3tubulin-specific sequences from...

The sequence of two human beta-tubulin pseudogenes is described. One contains an intervening sequence but lacks sequences encoding the 55 N-terminal amino acids of the polypeptide chain. A second has no introns but has a polyadenylate signal and an oligoadenylate tract at its 3' end, and it is flanked by a short direct repeat. These sequences have arisen by different mechanisms, including one that probably involves reverse transcription of a processed messenger RNA and reintegration of the complementary DNA copy into the genome.