Physical and Genetic Mapig of the Rhodobacter spaeroides Genome : Presence of Two Unique Circular Chromosoes

Abstract

,A. macrorestriction map ~epresenting the complete physical map of the Rhodobacter sphaeroides 2.4.1 ch~romosomes has been constrl~cted by ordering the chromosonial DNA fragments from total ge~,om;c DNA digested with the restriction endonucleases AseI, SpeI, DraI, and SnaBI. Junction fragments and multiple restriction endonuclease digestions of the chromosomal DNAs derived from wild-type and various mutant strains, in conjunction with Southern hybridization analysis, have been used to order all of the chromosomal DPJA fragments. Our results indicate that R. sphaeroides 2.4.1 carries two different circular chromosomes of 3,046 +- 95 and 914 2 17 kilobases (kb). Both chromosonie I(3,046 kb) and chromosome I1 (914 kb) contain rRNA cistrons. It appears th:~otn ly a single copy of the rRNA genes is contained on chromosome I (rrtzA) and that two copies are present on chron~osomeI 1 (rrnB, rrnC). Additionally, genes for glyceraldehyde 3-phosphate dehydrogenase (SapB) and ii-aminolevi~linic acid sgnthase (henzn are found on clironiosome 11. In each instance, there appears to be a second copy of each of these genes on chroniosonle I, but the extent of the DNA homology is very low. Genes giving rise to enzymes involved in CO, fixation and linked to the gene encoding the form I enzyme (i.e., the orm I region) are on chromosome I, whereas those genes representing the form I1 region are on chromosom: 11. The complete physical and partial genetic maps for each chrotnosome are presented.