Ectopic expression of the female transformer gene product leads to female differentiation of chromosomally male Drosophila

The transformer (tra) gene of Drosophila is necessary for all aspects of female somatic sexual differentiation. tra uses a single set of precursor RNAs to produce female- and non-sex-specific RNAs by alternative splicing. Ectopic expression of the female-specific RNA causes chromosomal males to develop as females, indicative of a linear pathway of regulated genes controlling sex. Genetic and molecular tests with this ectopically expressed gene are consistent with the following order of gene action: X chromosome to autosome ratio----Sex lethal----transformer----transformer-2----doublesex----intersex--- - terminal differentiation. Expression of the female-specific tra RNA in tra mutants is sufficient to lead to female differentiation. Expression of the non-sex-specific tra RNA in tra mutants is not sufficient to lead to female differentiation. The tra female-specific activity is not required for female-specific splicing of the tra precursor RNAs.     

Multicopy integration of heterologous genes, using the lactococcal group II intron targeted to bacterial insertion sequences

Group II introns are mobile genetic elements that can be redirected to invade specific genes. Here we describe the use of the lactococcal group II intron, Ll.ltrB, to achieve multicopy delivery of heterologous genes into the genome of Lactococcus lactis IL1403-UCD without the need for selectable markers. Ll.ltrB was retargeted to invade three transposase genes, the tra gene found in IS904 (tra904), tra981, and tra983, of which 9, 10, and 14 copies, respectively, were present in IL1403-UCD. Intron invasion of tra904, tra981, and tra983 allele groups occurred at high frequencies, and individual segregants possessed anywhere from one to nine copies of intron in the respective tra alleles. To achieve multicopy delivery of a heterologous gene, a green fluorescent protein (GFP) marker was cloned into the tra904-targeted Ll.ltrB, and the resultant intron (Ll.ltrB::GFP) was induced to invade the L. lactis tra904 alleles. Segregants possessing Ll.ltrB::GFP in three, four, five, six, seven, and eight copies in different tra904 alleles were obtained. In general, increasing the chromosomal copy number of Ll.ltrB::GFP resulted in strains expressing successively higher levels of GFP. However, strains possessing the same number of Ll.ltrB::GFP copies within different sets of tra904 alleles exhibited differential GFP expression, and segregants possessing seven or eight copies of Ll.ltrB::GFP grew poorly upon induction, suggesting that GFP expression from certain combinations of alleles was detrimental. The highest level of GFP expression was observed from a specific six-copy variant that produced GFP at a level analogous to that obtained with a multicopy plasmid. In addition, the high level of GFP expression was stable for over 120 generations. This work demonstrates that stable multicopy integration of heterologous genes can be readily achieved in bacterial genomes with group II intron delivery by targeting repeated elements.     

The sex-determining gene tra of Drosophila: molecular cloning and transformation studies.

In Drosophila, the primary signal for sex determination is given by the ratio of X chromosomes to sets of autosomes (X:A). The primary signal is read by a key gene (Sxl) and transmitted down to the differentiation genes by the subordinate control genes tra, tra-2, ix and dsx. Mutations in tra transform chromosomal females (X/X; tra/tra) into sterile males (pseudomales). We have cloned the tra region by microdissection and chromosomal walking. We identified the gene using deficiency breakpoints, DNA aberrations in three different alleles of tra and by P-mediated transformation. A 3.8-kb fragment perfectly rescued the mutant phenotype of X/X; tra/tra flies, showing that it contained all the necessary information to restore female-specific functions in the mutant flies. We present evidence that most of the function of tra can be provided by a subsegment of 2 kb that is differentially transcribed or processed in males and females.     

Genetic organization of transfer region of F-like plasmid pAP18-1

The results of complementation analysis of nitrosoguanidine-induced mutants of F-like drd-plasmid pAP18-1 (Tc, ColV) testified to the existence of at least 3 tra regions (tra1, tra2, tra3) and regulating locus fin V in the genome of this plasmid. By means of molecular cloning of tra2 region and locus fin V of plasmid pAP18-1drd were located in Sall-fragment f5 (3.9 MD).     

The transformer gene in Ceratitis capitata provides a genetic basis for selecting and remembering the sexual fate

The medfly Ceratitis capitata contains a gene (Cctra) with structural and functional homology to the Drosophila melanogaster sex-determining gene transformer (tra). Similar to tra in Drosophila, Cctra is regulated by alternative splicing such that only females can encode a full-length protein. In contrast to Drosophila, however, where tra is a subordinate target of Sex-lethal (Sxl), Cctra seems to initiate an autoregulatory mechanism in XX embryos that provides continuous tra female-specific function and act as a cellular memory maintaining the female pathway. Indeed, a transient interference with Cctra expression in XX embryos by RNAi treatment can cause complete sexual transformation of both germline and soma in adult flies, resulting in a fertile male XX phenotype. The male pathway seems to result when Cctra autoregulation is prevented and instead splice variants with truncated open reading frames are produced. We propose that this repression is achieved by the Y-linked male-determining factor (M).     

Sexual back talk with evolutionary implications: stimulation of the Drosophila sex-determination gene sex-lethal by its target transformer

We describe a surprising new regulatory relationship between two key genes of the Drosophila sex-determination gene hierarchy, Sex-lethal (Sxl) and transformer (tra). A positive autoregulatory feedback loop for Sxl was known to maintain somatic cell female identity by producing SXL-F protein to continually instruct the target gene transformer (tra) to make its feminizing product, TRA-F. We discovered the reciprocal regulatory effect by studying genetically sensitized females: TRA-F from either maternal or zygotic tra expression stimulates Sxl-positive autoregulation. We found female-specific tra mRNA in eggs as predicted by this tra maternal effect, but not predicted by the prevailing view that tra has no germline function. TRA-F stimulation of Sxl seems to be direct at some point, since Sxl harbors highly conserved predicted TRA-F binding sites. Nevertheless, TRA-F stimulation of Sxl autoregulation in the gonadal soma also appears to have a cell-nonautonomous aspect, unprecedented for somatic Sxl regulation. This tra-Sxl retrograde regulatory circuit has evolutionary implications. In some Diptera, tra occupies Sxl's position as the gene that epigenetically maintains female identity through direct positive feedback on pre-mRNA splicing. The tra-mediated Sxl feedback in Drosophila may be a vestige of regulatory redundancy that facilitated the evolutionary transition from tra to Sxl as the master sex switch.     

Interactions between sex-transformation mutants of Drosophila melanogaster. I. Hemolymph vitellogenins and gonad morphology

In Drosophila, vitellogenins (yolk protein precursors) are synthesized by the female fat body, secreted into the hemolymph and subsequently taken up by the developing oocytes. The male fat body, on the other hand, does not do this even when immature ovaries are transplanted into the body cavity and grow. Thus, the hemolymph vitellogenins serve as an easily detectable sexually dimorphic biochemical marker.--We have examined hemolymph vitellogenins by SDS polyacrylamide gel electrophoresis in flies carrying various sex-transformation mutants (dsx, tra, tra-2 and tra-2OTF) singly and in all possible combinations. Chromosomal females homozygous for tra or tra-2 have no detectable hemolymph vitellogenins, while those homozygous for tra-2OTF exhibit appreciable levels of these proteins. Flies homozygous for dsx, both X/X and X/Y, have hemolymph vitellogenins, although the amount is consistently smaller in the latter. Indeed, X/Y; dsx/dsx is the only genotype in which hemolymph vitellogenins are detected in the X/Y flies. A clear hierarchy of epistasis exists among these sex-transformation mutants when they are examined in various combinations: dsx greater than tra, tra-2 greater than tra-2OTF. Moreover, an interaction between tra-2OTF and tra was seen in these experiments: X/X; tra-2OTF/tra-2OTF flies show the presence of only a trace of hemolymph vitellogenins when they are made heterozygous for tra. These results, combined with observations on gonad morphology, are discussed with respect to the Baker and Ridge (1980) hypothesis of sex determination.     

Molecular Population Genetics of Sex Determination Genes: The Transformer Gene of Drosophila Melanogaster

The transformer locus (tra) produces an RNA processing protein that alternatively splices the doublesex pre-mRNA in the sex determination hierarchy of Drosophila melanogaster. Comparisons of the tra coding region among Drosophila species have revealed an unusually high degree of divergence in synonymous and nonsynonymous sites. In this study, we tested the hypothesis that the tra gene will be polymorphic in synonymous and nonsynonymous sites within species by investigating nucleotide sequence variation in eleven tra alleles within D. melanogaster. Of the 1063 nucleotides examined, two synonymous sites were polymorphic and no amino acid variation was detected. Three statistical tests were used to detect departures from an equilibrium neutral model. Two tests failed to reject a neutral model of molecular evolution because of low statisitical power associated with low levels of genetic variation (Tajima/Fu and Li). The Hudson, Kreitman, and Aguade test rejected a neutral model when the tra region was compared to the 5'-flanking region of alcohol dehydrogenase (Adh). The lack of variability in the tra gene is consistent with a recent selective sweep of a beneficial allele in or near the tra locus.     

Characterization of Pseudomonas aeruginosa derepressed R-plasmids.

A genetic study of conjugal transmissibility of two R-plasmids was undertaken in Pseudomonas aeruginosa. Conjugally derepressed mutants of the R-plasmids were isolated, and examination of 11 independent mutants revealed that 10 were recessive to the wild-type transfer repressor, whereas 1 mutant was cis dominant. Cross-repression was observed between the two R-plasmids, suggesting that they have functionally equivalent systems for regulating the expression of tra loci. The derepressed R-plasmid mutants exhibited several characteristics, in addition to derepressed transfer, that were not expressed by the parental plasmids. These included sensitivity to certain donor-specific phages, inhibition of multiplication of a transducing phage, and, in the one case examined, a high degree of entry exclusion. The coexpression of these different functions suggests that their respective genetic loci are controlled by the same regulatory system as that of tra, or else that they are part of the tra complex.     

lambda-Transducing phages carrying transfer genes isolated from an abnormal prophage insertion into the traY gene of F

A set of lambda-transducing phages carrying transfer (tra) genes has been isolated from an abnormal lysogen in which a lambda prophage was inserted into the traY gene of Flac. These have been characterized genetically for complementation of Flac tra and finP point mutants and for the presence of oriT. Studies of tra gene expression during lambda repression showed that tra genes on the transducing phages were expressed from the lambda PL promoter as well as from the transfer promoters when these were present. The molecular weights of the traM (14,000) and traJ (23,500) proteins were measured after infection of ultraviolet-irradiated cells with one of the phages, ED lambda 102, and overproduction of the traJ protein upon induction of an ED lambda 102 lysogen was demonstrated. A proportion of this traJ protein was located in the inner membrane and cytoplasmic fractions of the cell, the majority being in the outer membrane. Physical analysis of the DNA carried by the lambda tra phages by determination of the phage buoyant densities in CsCl, by restriction enzyme digestion and by electron microscope heteroduplex analysis, was used to define the DNA segments encoding the tra functions. Correlation of the physical and genetical data improved the positioning of the tra genes within the transfer region. These results were combined with new restriction enzyme cleavage data to construct an improved map of this region.     

Identification and cloning of the conjugative transfer region of Staphylococcus aureus plasmid pGO1.

The conjugative transfer (tra) genes of a 52-kilobase (kb) staphylococcal plasmid, pGO1, were localized by deletion analysis and transposon insertional inactivation. All transfer-defective (Tra-) deletions and Tn551 or Tn917 transposon insertions occurred within a 14.5-kb BglII fragment. Deletions and insertions outside this fragment all left the plasmid transfer proficient (Tra+). The tra region was found to be flanked by directly repeated DNA sequences, approximately 900 base pairs in length, at either end. Clones containing the 14.5-kb BglII fragment (pGO200) and subclones from this fragment were constructed in Escherichia coli on shuttle plasmids and introduced into Staphylococcus aureus protoplasts. Protoplasts could not be transformed with pGO200E (pGO200 on the staphylococcal replicon, pE194) or subclones containing DNA at one end of the tra fragment unless pGO1 or specific cloned tra DNA fragments were present in the recipient cell. However, once stabilized by sequences present on a second replicon, each tra fragment could be successfully introduced alone into other plasmid-free S. aureus recipients by conjugative mobilization or transduction. In this manner, two clones containing overlapping fragments comprising the entire 14.5-kb BglII fragment were shown to complement each other. The low-frequency transfer resulted in transconjugants containing one clone intact, deletions of that clone, and recombinants of the two clones. The resulting recombinant plasmid (pGO220), which regenerated the tra region intact on a single replicon, transferred at frequencies comparable to those of pGO1. Thus, all the genes necessary and sufficient for conjugative transfer of pGO1 are contained within a 14.5-kb region of DNA.     

Regulation of sexual differentiation in D. melanogaster via alternative splicing of RNA from the transformer gene

The transformer (tra) gene regulates female somatic sexual differentiation and has no known function in males. It gives rise to two sizes of RNA, one non-sex-specific and one female-specific. These two RNAs are shown to be present throughout the life cycle, and related by the use of alternative first intron splice acceptor sites. The non-sex-specific RNA has a 73 base first intron, while that in the female-specific RNA is 248 bases. The non-sex-specific RNA has no long open reading frame, while the female-specific RNA has a single long open reading frame beginning at the first AUG. Substitution of a heat shock promoter for the tra promoter still leads to female-specific differentiation of otherwise tra-females. We suggest a mechanism by which Sex-lethal controls itself and tra.     

Conjugational complementation analysis of transfer-deficient mutants of Flac in Escherichia coli

A series of 102 transfer-deficient (tra(-)) mutants of Flac (84 of which have been previously described) were classified as carrying frameshift, amber, ochre, UGA, and nonsuppressible mutations. Techniques were evolved for using cultures of F prime strains as efficient recipients in matings, for measuring the number of (F'/F') heterozygote cells in transient populations, and for measuring complementation between different Flac tra(-) mutants. These techniques were used to define nine tra cistrons and to assign most of the tra(-) mutations to one or other of these.