Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

We have constructed a mammalian expression vector consisting of 3 kilobases of the human fi-actin gene 5' flanking sequence plus 5' untranslated region and intervening sequence I linked at the 3' splice site to a short DNA polylinker sequence containing unique Sal I, HindIII, and BamHI restriction endonuclease sites followed by a simian virus 40 (SV40) polyadenylylation signal. Two derivatives, containing the selection markers obtained from pSV2gpt or pSV2neo, were also generated. We find that the promoter activity of this vector is as great or greater than that of the SV40 early promoter in a variety of human and rodent cells. The vector was used to generate y-actin and 8-tubulin antisense transcripts in human fibroblast cell lines. The antisense transcripts accumulate to levels comparable with that of the highly abundant y-actin and .-tubulin mRNAs.

We report the complete nucleotide sequence of a human ,B actin cDNA. Both the 5' and 3' untranslated regions of the sequence are similar (>809%) to the analogous regions of the rat 8-actin gene reported by Nudel et al (1983). When a segment of the 3' untranslated region is used as a radiolabelled probe, strong hybridization to chick I8 actin mRNA is seen. This conservation of sequences suggests that strong selective pressures operate on non-translated segments of,B actin mRNA.