Subroutines in the programme of Chlamydomonas gene expression induced by flagellar regeneration

The unicellular green alga Chlamydomonas reinhardtii possesses two anterior flagella that are rapidly replaced if they are lost. A cytoplasmic pool of flagellar precursors supports regeneration of partial length flagella, but complete regeneration requires de novo synthesis of flagellar proteins. This increase in protein synthesis is transient and is programmed by changes in the physical abundance of a set of RNAs. These changes were measured using cloned cDNAs. The curves that were generated with probes for a number of different RNAs were variations-on-the-theme of a rapid accumulation following deflagellation of the cells, followed by similarly rapid degradation. Differences in the characteristics of the accumulation curves suggest that several 'subroutines' for RNA metabolism appear to run concurrently during flagellar regeneration. A significant portion of the mRNA abundance regulation occurs at the transcriptional level. A group of cDNAs that encode mRNAs whose abundance remains at constant levels during regeneration provides an important internal control for the abundance and transcription measurements. Flagella contain over 200 different proteins, so the possibility exists that the cells coordinately and rapidly alter the transcription of the same number of genes. Chlamydomonas flagellar regeneration thus provides an opportunity to study cellular mechanisms for coordinating the expression of a large set of genes, and correlating these changes with an easily monitored morphogenetic process.