Expression and struture of the CyIIIb actin gene of the sea urchin Strongylocentrotus purpuratus

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Abstract
The developmental pattern of expression of the Strongylocentrotus purpuratus Cylllb actin gene was determined by RNA blot hybridizations carried out with a gene-specific probe and total embryonic RNA isolated from various stages of development. The results indicate that the Cylllb mRNA is not detected in the maternal pool, and, although the gene is activated at the early stages (about 10 hr postfertilization), considerable amounts of mRNA do not accumulate until well into the pluteus stage 3 days later. These results suggest either a post-transcriptional regulatory mechanism that governs early embryonic expression of the Cylllb actin or a late embryonic transcriptional enhancement of this gene. We present here the complete nucleotide sequence of the Cylllb gene, which lies within the 10,361 base pairs of the sequenced region. The entire transcription unit is 7,455 nt long and shares structural similarities with the other cytoskeletal-type actin genes from this sea urchi. Sequence comparisons of Cylllb to the Cyllla actin gene, to which it is linked, reveals extensive homology even in the introns. The deduced amino acid sequence of the Cylllb actin shows five amino acid substitutions compared with the Cyllla actin and nine when compared with the Cyl, the endodermal embryonic cytoskeletal-type actin. Five out of these nine amino acid differences occur within a small peptide (position 257 to 267). The 5′ flanking sequence of the Cylllb gene shows a remarkable homology (∼75–80%) with the Cyllla upstream region up to the position −200 and a lack of any obvious similarity further upstream. This observation suggests that the two genes possibly share some common regulatory factors.