longitudinals lacking (lola) is a complex Drosophila gene encoding at least 20 protein isoforms, each bearing the same N-terminal constant region linked to a different C-terminal variable region. Different isoforms specify different aspects of axon growth and guidance. We show here that lola mRNAs are generated by alternative trans-splicing of exons sequentially encoded by the same DNA strand. Chromosomal pairing facilitates interallelic trans-splicing, allowing complementation between mutations in the constant and those in the variable exons. We demonstrate that at least one variable exon is transcribed from its own promoter, and transspliced to the constant exons transcribed separately.Supplemental Material is available at http://www.genesdev.org.

Alternative splicing is an important process contributing to proteome diversity without involving an increase in the number of genes. In some cases, alternative splicing is carried out under 'trans-mode', called alternative trans-splicing, in which exons located on separate pre-mRNA molecules are selectively joined to produce mature mRNAs encoding proteins with distinct structures and functions. However, it is not known how widespread or how frequently trans-splicing occurs in vivo. Recently, trans-allelic trans-splicing has been unambiguously demonstrated in Drosophila using a SNP (single nucleotide polymorphism) as a marker. In this review, we provide an overview of alternative trans-splicing in Drosophila and mammals, and discuss its mechanisms. IntroductionAlternative splicing is one of the processes by which a single gene produces distinct mRNAs through differential selection of potential splice sites, providing a crucial source of diversity in the proteomes of higher eukaryotes. Splicing can occur either in cis or in trans: cis-splicing joins exons within a premRNA molecule, whereas trans-splicing joins exons on separate pre-mRNAs. Trans-splicing commonly occurs in unicellular organisms and Caenorhabditis elegans. In these organisms, genes are clustered and transcribed polycistronically. These pre-mRNAs are processed into monocistronic mRNAs, whose 5 ends are formed by trans-splicing of SL (spliced leader) RNAs that are transcribed separately. SL RNA transsplicing has been studied extensively, including its functional significance (reviewed in Blumenthal and Steward, 1997;Nilsen, 1997). However, SL RNA trans-splicing does not contribute to proteomic diversity, since the SL exon is a non-coding sequence. Recently, alternative trans-splicing has been discovered in Drosophila and mammals, and in some cases has been shown to be physiologically significant.

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Two genes, fruA and csgA, encoding a putative transcription factor and C-factor, respectively, are essential for fruiting body formation of Myxococcus xanthus. To investigate the role of fruA and csgA genes in developmental gene expression, developing cells as well as vegetative cells of M. xanthus wild-type, fruA::Tc, and csgA731 strains were pulse-labeled with [35 S]methionine, and the whole cell proteins were analyzed using two-dimensional immobilized pH gradient/SDS-PAGE. Differences in protein synthesis patterns among more than 700 protein spots were detected during development of the three strains. Fourteen proteins showing distinctly different expression patterns in mutant cells were analyzed in more detail. Five of the 14 proteins were identified as elongation factor Tu (EF-Tu), Dru, DofA, FruA, and protein S by immunoblot analysis and mass spectroscopy. A gene encoding DofA was cloned and sequenced. Although both fruA and csgA genes regulate early development of M. xanthus, they were found to differently regulate expression of several developmental genes. The production of six proteins, including DofA and protein S, was dependent on fruA, whereas the production of two proteins was dependent on csgA, and one protein was dependent on both fruA and csgA. To explain the present findings, a new model was presented in which different levels of FruA phosphorylation may distinctively regulate the expression of two groups of developmental genes.

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