Modulation of muscle gene expression in Caenorhabditis elegans: differential levels of transcripts, mRNAs, and polypeptides for thick filament proteins during nematode development.

The body-wall muscle cells of the nematode Caenorhabditis elegans produce thick filaments during embryonic, larval, and adult stages. These thick filaments contain two myosin isoforms, A and B, which assemble into different zones along the 10-microns lengths. Paramyosin, a protein homologous to myosin rods, forms a substratum for the myosins. The three filament proteins are encoded by different genes: myo-3 V (myosin heavy chain A), unc-54 I (myosin heavy chain B), and unc-15 I (paramyosin). The relative expression of these genes has been studied by run-on nuclear transcription in vitro, hybridization of accumulated mRNA, and immunochemical determination of specific polypeptide accumulation. In late larval nematodes (L4), the relative levels of nuclear run-on transcription per mol of probe are 6.4 unc-54:2.4 myo-3:1.0 unc-15. Similarly, the relative levels of immunospecific proteins are 4.5 unc-15:3.1 unc-54:1.0 myo-3. Most strikingly, the relative mRNA amounts are 50.0 unc-54:12.4 unc-15:1.0 myo-3. Thus, the orders of relative abundance and the quantitative relations of expression of the three functionally related genes change from transcriptional activities to final accumulated product of thick filament proteins. Modulation of the expression appears to involve processes affecting accumulation of mRNA and protein. The great difference in accumulation of the mRNAs for the two myosin heavy chain isoforms A and B may be related to the different roles of the myosins in thick filament assembly.