WHEN YEAST'S chromosome III became the first chromosome to have its DNA described fully, a big surprise lay in store for scientists: They stumbled upon a wealth of genes whose functions are completely unknown. "All of a sudden we learn there is a whole class of genes, more than half the genome, of which we were totally ignorant," said biochemist Andre Goffeau of the University of Louvain in Belgium, in Science (Vol. 256, No. 5058), who coordinated the European Community (EC) project that worked out the sequence. A genome is the entire length of the genetic letters or nucleotide bases that make up a chromosome.

Biologists have no idea what the proteins made by these genes actually do. In fact, only three of 55 novel genes that were tested appeared to be absolutely essential for life. This was demonstrated by deleting each of the genes in turn to find out how it affected the yeast. Yeast cells and human cells are more or less similar, except that the former contains just 0.5 per cent of the DNA of the human cell. Scientists believe that by studying yeast, they would be able to learn more about vital genes.

Yeast-type genes often turn up in other organisms, performing identical functions as in yeast. For example, one of the new genes was found to be homologous, or similar, to one found in nitrogen-fixing bacteria. However, in most cases where a gene does the same job in different organisms, it is still not clear how the job is done. Scientists say it should be much easier to work this out in yeast, than in other organisms for the simple reason that yeast chromosomes are one of the smallest and contain much less DNA.

But investigating the function of the new genes will be time-consuming and far tougher than the original sequencing. "With genes of totally unknown function, it is of course difficult to decide what conditions to look for. One can look for obvious characteristics like morphology and temperature sensitivity, but the list can never be exhaustive," noted Piotr Slonimski, project coordinator for functional analysis and director of the Centre de Genetique Moleculaire at Gif-sur Yvette in France.