THE CHLAMYDOMONAS DHC1 GENE ENCODES A DYNEIN HEAVY-CHAIN SUBUNIT REQUIRED FOR ASSEMBLY OF THE I1 INNER ARM COMPLEX

Multiple members of the dynein heavy chain (Dhc) gene family have been recovered in several organisms, but the relationships between these s equences and the Dhc isoforms that they encode are largely unknown. To identify Dhc loci and determine the specific functions of the individ ual Dhc isoforms, we have screened a collection of motility mutants ge nerated by insertional mutagenesis in Chlamydomonas. In this report, w e characterize one strain, pf9-3, in which the insertion event was acc ompanied by a deletion of similar to 13 kb of genomic DNA within the t ranscription unit of the Dhc1 gene. Northern blot analysis confirms th at pf9-3 is a null mutation. Biochemical and structural studies of iso lated axonemes demonstrate that the pf9-3 mutant fails to assemble the I1 inner arm complex, a two-headed dynein isoform composed of two Dhc s (1 alpha and 1 beta) and three intermediate chains. To determine if the Dhc1 gene product corresponds to one of the Dhcs of the I1 complex , antibodies were generated against a Dhc1-specific peptide sequence. Immunoblot analysis reveals that the Dhc1 gene encodes the 1 alpha Dhc subunit. These studies thus, identify the first inner arm Dhc locus t o be described in any organism and further demonstrate that the 1 alph a Dhc subunit plays an essential role in the assembly of the I1 inner arm complex.