A FLAGELLAR CALMODULIN GENE OF NAEGLERIA, COEXPRESSED DURING DIFFERENTIATION WITH FLAGELLAR TUBULIN GENES, SHARES DNA, RNA, AND ENCODED PROTEIN-SEQUENCE ELEMENTS

Two calmodulins are synthesized during differentiation of Naegleria gr uberi from amoebae to flagellates; one remains in the cell body and th e other becomes localized in the flagella, The single, intronless, exp ressed gene for flagellar calmodulin has been cloned and sequenced, Th e encoded protein is a typical calmodulin with four putative calcium-b inding domains, but it has an amino-terminal extension of 10 divergent amino acids preceding conserved calmodulin residue 4. The transcripts encoding flagellar calmodulin and flagellate cell body calmodulin are clearly divergent, Expression of the flagellar calmodulin gene is dif ferentiation specific; its mRNA appears and then disappears concurrent ly with those encoding flagellar alpha- and beta-tubulin, Three provoc ative sequence elements are shared among these unrelated coexpressed g enes: (i) a palindromic DNA sequence element is found in duplicate or triplicate upstream to each transcribed region; (ii) a perfect 12-nucl eotide match is found near the AUG start codon of flagellar calmodulin and alpha-tubulin; and (iii) the novel amino-terminal extension of fl agellar calmodulin contains a 5-amino-acid element similar to the amin o terminus of flagellar alpha-tubulin, These shared sequence elements are proposed to have roles in differentiation, possibly in regulation of transcription, mRNA stability, and localization of these proteins t o flagella.