CHLAMYDOMONAS INNER-ARM DYNEIN MUTANT, IDA5, HAS A MUTATION IN AN ACTIN-ENCODING GENE

Chlamydomonas flagellar inner-arm dynein consists of seven subspecies (a-g), of which all but f contain actin as subunits. The mutant ida5 a nd a new strain, ida5-t, lack four subspecies (a, c, d, and e). These mutants were found to have mutations in the conventional actin gene, s uch that its product is totally lost; ida5 has a single-base deletion that results in a stop codon at a position about two-thirds from the 5 ' end of the coding region, and idn5-t lacks a large portion of the en tire actin gene. Two-dimensional gel electrophoresis patterns of the a xonemes and inner-arm subspecies b and g of ida5 lacked the spot of ac tin (isoelectric point [pI] = similar to 5.3) but had two novel spots with pIs of similar to 5.6 and similar to 5.7 instead. Western blot wi th different kinds of anti-actin antibodies suggested that the protein s responsible for the two novel spots and conventional actin are diffe rent but share some antigenicity. Since Chlamydomonas has been shown t o have only a single copy of the conventional actin gene, it is likely that the novel spots in ida5 and ida5-t originated from another gene( s) that codes for a novel actin-like protein(s) (NAP), which has hithe rto been undetected in wildtype cells. These mutants retain the two in ner-arm subspecies b and g, in addition to f, possibly because NAP can functionally substitute for the actin in these subspecies while they cannot in other subspecies. The net growth rate of ida5 and ida5-t cel ls did not differ from that of wild type, but the mating efficiency wa s greatly reduced. This defect was apparently caused by deficient grow th of the fertilization tubule. These results suggest that NAP can car ry out some, but not all, functions performed by conventional actin in the cytoplasm and raise the possibility that Chlamydomonas can live w ithout ordinary actin.